Symptoms and diagnosis:
"Mary Sue" is a term in literary analysis/nitpicking that is something like "paranoia" or "neurosis" in psychology: its original narrow (and useful) meaning has been largely swallowed in an overbroad application.
Originally a "Mary Sue" was a character in fan fiction who was transparently the writer living out an imagined-to-be-perfect existence in the fictional world of which the writer was a fan. The stereotype was female, perhaps because the stereotypical fanfic writer was female*.
The classical Mary Sue is easy to parodize in other people's work, depending on your mood and how much you like cruelty to the defenseless. It might be Euphoria Understanden, Sherlock Holmes's perfect girlfriend who also happened to have twenty cats, loved to sip tea and listen to Vivaldi, and was really too sensitive to be working in a bookstore where people made her carry heavy boxes, and was always there to add that little human feminine touch to Holmes's reasoning. It could be Gagaliol Eaglesfriend Baggins, who married Frodo Baggins after he went west, because it turned out that old Frodo had a thing for a well-read elf with meat on her bones. Less commonly it might be Ensign Studly, the bodyguard assigned to Deanna Troi, who fixed everything for her, rubbed her back after a long day, provided sensitive insights into her most difficult cases, and then made her forget all about all those macho guys.**
Back when Mary Sues were purely wish-fulfillment in borrowed universes -- in effect making the perfect world the fan wanted to move into just that bit more perfect -- the charge of Mary-Sue-ism was a slam dunk in the literary prosecutor's arsenal. Narrow-sense Mary Sues were somewhat embarrassing, and the solution to them was to just not do them, and if people got caught at them it tended to be painful for everyone.
But the universe has marched on and workshops have become an industry. The compulsion to say something (especially when there is nothing to say other than "keep going, it's okay") drives too many workshoppers. The aggressive colonizing of reader mindspace by workshopper tropes also shows up in the slow creep of workshopese into Goodreads and Amazon reviews, especially among people who have a hard time differentiating between writing that didn't please them and writing that made technical errors. (If you don't like chicken, you won't like chicken tetrazzini , but it will not help you find out what you do like, or the writers to please you better, if you tell them that they broke all the rules for making pork lo mein).
As a result, "Mary Sue" has become short hand for something rather like "I didn't like that character" (especially that main character) "because she was just too impossibly good, virtuous, competent, strong … " and the list goes on. Too much gooditude spoiled the character, at least for that critic/reader.
At first this may seem a bit odd in a culture that not only invents huge numbers of superheroes (literally or figuratively) and also has something of a fascination with the inverse of super-heroes, monsters of one kind or another (ranging from real ones like serial killers to the literal ones like Dracula all the way up to the archetypal ones like Satan). The overbroad definition of Mary Sue, a character who is "just too good" or "just too perfect" sounds something like the complaint that Mozart wrote too many notes, Van Gogh's colors were too vivid, or that it's hard to figure out Shakespeare's or Tolstoy's characters.
I suggest that Mary Sue is the lit-crit equivalent of cellulite, i.e. it's not actually a health of the story issue, it's something that a few people find unattractive and will pick on you for, and to a great extent you shouldn't let the mean girls (of whatever gender) make you self-conscious; a large part of the fix is just to realize that tons of people will love your story even if it's got a little cottage cheese on its muscular thighs.
Quack remedy and why to stay away from it:
Like any problem that is largely illusory and a matter of making the recipient of the criticism feel unloved and ugly, Mary Sue-ism carries with it some popular patent remedies. One of them is the frequent demand that you "dirty up" the main character, and make them "gritty," whether it's really appropriate or not.
In fact, many writers who haven't read enough of their intended genre – and there are more of those than you would think, and sadly some of them get pretty far in publishing before it catches up with them – seem to be reacting to a morally pure universe in pop fiction that hasn't been there in a long time; I've seen work at the close-to-publishable stage from people who seem to think they're "breaking through" by including an angsty hero with an unpleasant character defect. I have actually heard a would-be writer say he wanted to create a character who would be "like Batman but not such a goody-goody Boy Scout." (Yes, I suggested he read anything about Batman published in the last thirty years; he didn't think he needed to because everyone knows Batman. Oh, well. Horse, water, drink, etc.)
Then too, the same workshoppers who feel compelled to say something (whether or not they have any idea what to say) very often have some formulas that nearly always work, and complaining that the hero/ine is "too goody-good" or "too super-competent" may simply be a way for a critiquer out of depth to conceal his/her limitations. Maybe you just need to stop going to that particular workshop, or to stay home and write.
Of course one reason the quack remedy remains popular is that frequently, introducing a defect, or seven, into a character does improve the character, not because it de-Mary-Sue-ifies the character but just because the defect itself is interesting to read about, or makes the agon that is central to the plot more challenging to the character. Let us suppose you have written a story with a too-centered-on-a-fantasy-of-perfection heroine: a natural flame-redhaired perfect-skinned werewolf vegan vampire reformer (she works in a no-slay secret organization with a charter that fully respects undead rights) who volunteers on the crisis hotline. Almost any personal problem would make that character a bit more likeable and interesting, and if you can invent a problem that makes her most important fictional task harder, you'll genuinely have improved the work. Just because a lot of over the counter stuff is overhyped and ineffective doesn't mean aspirin won't fix your headache.
Maybe she was cheating on her boyfriend when her co-cheater turned out to be a vampire and bit her, and now she's got till sundown to find a very liberal priest to do an exorcism, and she needs the vampire's fangs for the ritual, and you normally get a vampire's fangs by picking them up after slaying him, and worse yet, she really doesn't like her boyfriend anymore anyway ... that sort of complexity can give you at least some sort of plot. Binge-drinking might not, and an uncle with early Alzheimer's probably not either. (Though if you have to watch your uncle with early Alzheimer's and fight vampires and your long-dead aunt is a vampire that he would let into the house in a heartbeat (his last)... hmmm. It's surpisingly hard to write examples of genuinely impossible fictional ideas).
But usually my book doctoring clients have been long past such easy problems that have such easy fixes. Too often "dirty that character up" is kind of a Band-aid on a story, hiding but not healing a raw spot, and at best allowing you to focus your attention on something more important. So if that's the first suggestion you get, either from a reader or from your own subconscious, don't ignore the possibility, but think things over first; maybe there's more to it.
Which brings us to: When Mary Sue is serious, and a little grit won't fix her
Mary Sue might not be a symptom, but the complaints about Mary Sue-ism might be; if people are seeing her, it might be all them, or partly them, but there might be a part that is you, and that's what you want to chase down and repair.
As a book doctor, I prefer deeper fixes with more pervasive effects, and it has seemed to me that stories that are indicted for Mary Sue-ism are often suffering from a problem that is much more serious, interesting, and worthwhile.***
Mary-Sueism may appear exactly because you're trying to do something hard. Quite possibly your impossibly goody-good character has popped into your draft because material for a much better story is frantically signaling its presence, and you wrote Lady Perfection or Captain Paragon while you were distracted in digging for that vein of gold. If you can connect to that difficult material or idea, whatever it is, and let it take over your story, you'll ultimately be much happier with the results.
The single reader reaction of "that's a Mary Sue" can mask several different causes. Your workshoppers, editor, trusted beta readers, or agent are probably on to something, even if it's not the apparent Mary Sue, and your problem is to locate that real difficulty, and center your story's implied artistic effort on solving that. In such a case, you have a starting place for something that may eventually be really excellent, and Mary Sue pointed you to it. So don't throw mud on her perfect white dress until you're sure she's got it coming. She might be on your side. Here are five interesting places she might lead you:
Possible hidden cause of the Mary Sue complaint #1: s/he feels manipulative; s/he looks like a key prop in some little sermon that the writer would really rather give you instead of this story. Some writers do have the idea that they are going to put some religious/moral/political message out there with a clever story that is going to be a best-seller, and unfortunately some of the ones who want to do that happen to be natural-born clever storytellers, so they get a long way, all the way to an editor's or agent's "maybe" pile, before their intended hostile takeover of the reader's belief system finally sets off too many bells, and the book goes off to the book doctor. Possible remedies:
a. Stop preaching and tell a damned story. It's a higher calling anyway. Surprising numbers (as in, two) of my clients loved writing fiction, were born to do it, but were attached to a person (a husband, a mother) who did not approve of fiction for its own sake, and to get that screeching "You are wicked to spend time on this foolish pursuit" monkey off their backs, the writers were trying to write fiction that trotted out the favorite hobbyhorses of their fiction-hating loved ones.
b. Get honest about how tough it is to live according to any principle. If Mary Sue absolutely has to be a Christian model of virginity till marriage, she needs to be soaking her pants every time she smells her boyfriend's cologne and avoiding the cucumbers in the produce aisle, and all the thou shalt nots should be driving her crazy, not making her happy. If Captain Paragon will never ask a man to do something he wouldn't do, or leave a man behind, give him acute claustrophobia and no volunteers when he asks for who will go back into the collapsing mine to either confirm that Private Friendless McAsshole is dead or bring him out alive.
Possible hidden cause of the Mary Sue problem #2: s/he violates the sympathetic contract; despite the drippy little whines that you hear from too many people, there's no requirement that a character be admirable or likeable, but there is a requirement that the character be interesting. Clause one in the sympathetic contract is, Give this work your attention, follow this guy/girl that the work is following, and sooner or later something interesting will happen.**** In a good sympathetic contract, sometimes there's a fun diversion of attention, so that you follow a character for one reason and then are delighted to find out that there's was something else entirely interesting – we follow Percy Blakeney because it's funny that a brainy spitfire like Marguerite has married a foppish tool like Percy, until the moment when, with nowhere else to turn, she tells him about the menacing French agent – and then we laugh in a different way when it turns out that his apparent brainlessness was all an act and he's actually the Scarlet Pimpernel. Marion Crane (Janet Leigh's role in Psycho) looks like the focal character of a caper movie till she abruptly turns into the victim of a monster movie; that stretches the sympathetic contract in a fun way but doesn't break it.
But Mary Sue starts out too perfect to believe, losing all interest because perfection is predictable, and then stays that way. We followed her on the writer's promise that she would become interesting, and when she obstinately didn't, we hate her, the story, and the writer. Scarlett O'Hara needs to do everything that a proud Southern belle will never do to save Tara, Clark Kent needs to give up his amiable bumbling and rip his shirt open, and
Spoiler ===> Captain Reynaud needs to find his patriotism and cover up Major Strasser's murder. Nor does it have to be a reversal: in High Noon, Spoiler ===> smugly righteous pacifist Amy discovers her real principles when she shoots Frank Miller to save her husband's life, and in The Man Who Shot Liberty Valance, Spoiler ===> John Wayne coldbloodedly shoots a man in the back without warning, and destroys all his own hopes for future happiness, because he's a Code-of-the-West moral paragon. The terrifying irony of The Postman Always Rings Twice is that (this isn't really a spoiler, the book is much more about how it happens than about what happens) a sniveling little shit who has no problem with murdering a man who has always been kind to him in order to get his friend's sleazebag wife for himself eventually goes to the gas chamber because he and his lover are too sleazy and petty to hold together their perfect murder.
In all those cases, the sympathetic contract works with, not despite, excessive character consistency. The sympathetic contract demands a payoff for monotonic goodness. (and one reason I ended up flagging so many spoilers above is that payoffs tend to be potential spoilers). The reader will tolerate, even love, your story as long as perfection (in good or evil) leads to an "of course!" surprise. Hence the remedy: pay up. Put the payoff in. (I hasten to add, having Mary Sue die tragically after a last kiss with the main character is NOT such a payoff).
Possible hidden cause of the Mary Sue problem #3: s/he drenched in "should," which we don't feel. We want to feel "is".
Sometimes the Mary Sue complaint is phrased as "just too big for life."
The nature of life is that the unbelievably talented don't think they're unbelievable, they just use their talents. The incredibly virtuous think everyone would have done the same thing. Maybe your Mary Sue is too aware that she's amazing, and all you need is to insert a gentle undertone of "doesn't everyone?"
After all, who's bigger than life? Robin Hood. Lancelot. Kip in Have Spacesuit Will Travel. Hap and Leonard in Joe Lansdale's terrific mysteries. Scarlett O'Hara. Captain Peter Blood. D'Artagnan. Hercules.
Why aren't they Mary Sues? Because they have a force-of-nature quality about them – they just are that way, this is who they are and what they do, and to some extent they are matter of fact about it. D'Artagnan knows that he's a natural wizard with a sword, so fighting three duels in three hours against the cream of the King's Musketeers … piffle, of course, that's who he is. Scarlett O'Hara can't conceive of any man she meets not being locked on getting under her hoop skirt (which is why putting her up against Rhett works so well). Robin Hood never shoots without being sure of hitting his target, and when Ben Grimm decides it's clobbering time he doesn't wonder whether he'll be the clobberer or the clobberee. Roy Rogers doesn't think about whether, maybe this time, he'll take a big bribe from the crooked cattle baron and let the struggling ranch family go under. A character that is good (in either the sense of talent or virtue) is a totally different thing from a character that ought to be good for the message, needs to be good for the plot, or exemplifies good.
Remedy: get it back to
I ain't braggin', it's understood,
Everything I do, I sure do it good.
and lose any traces that your character is being good (moral/competent) for any reason other than that's just how they are. (Big hint, though: a character who is good both ways is hard to pull off. Not impossible -- Superman, Green Lantern, and the Flash in their traditional versions certainly had a good long run—but in a general way a character who knows absolutely what's right will benefit from some doubt about his/her ability, and a character who can always do whatever is needed should have some deep doubts about what that is).
Possible hidden cause of the Mary Sue problem #4: s/he is a winner as imagined by a loser. This one's touchy, and for the love of all that's holy please don't sling this around in workshop and maul the other members, but nonetheless I've seen it with several clients.
But this is just you and me and the little black bag, right? Look around and make sure no one is looking over your shoulder.
Okay, now trake a deep breath and face up to things here. Is your experience of success limited, far in the past, or nonexistent? Have you felt "no good at anything" for as long as you can remember? Have you never won a game, gotten the much-sought-after romantic partner, set a record, made the final bracket?
And if so, is what you imagine success would be like the emotional core of your Mary Sue? Natural Charlie Browns (as in Peanuts, not the several others in literature, arts, and sports), the Omegas of the pack who would love to just be low Betas but can't imagine rising that far, usually have very odd notions of what it's like to succeed, sort of like anorgasmic virgins trying to imagine sex, people who have never traveled imagining being seasoned travelers, people who have never been in any kind of fight imagining violence, etc.
Please note, some people can write very well about experiences they have not had (and even ones for which they have had no analogs). See The Red Badge of Courage as one example, or Len Deighton's WW2 aviation novels for another. Some others can produce a version more convincing than real life: successful Westerns are written all the time by people who have never ridden a horse.
But some people can't imagine some experiences without having been there, and some of the most difficult things to describe without having been there are psychological/ relational/cultural. It wasn't until quite recently in my own life that I had any idea what it was like to be in an affectionate, supportive family; I wrote functional families in short superficial scenes, or I wrote about orphans, or when backed into a corner I just plain wrote unconvincingly. I worked with one writer whose work took a dramatic turn for the better because in the REBT that he was using to cope with his Asperer's, he had the breakthrough of realizing that most people do have an idea of what the person they're talking to is feeling, and thus there started being real characters in his stories in addition to the viewpoint character.
I think, based on three problems I've seen with apparent Mary Sues, that success is like that for some people. Their Mary Sues are perfect and smug because the writers are imagining something better than they've ever had, and it's not only not convincing, it's about the wrong things, a little like the way that men who don't know very many women well will write female characters who think about their own breasts all the time (because that's the thing the man thinks about, so he imagines that women, in the constant presence of real, actual boobs, must be unable to stop thinking about them).
To many perpetual losers, winning would mean being able to sneer at the former champion, or be effusively condescending or improbably supportive to the loser, or even just enjoying the absence of pain far more than people do in real life. (If you had a left knee that ached ceaselessly from birth, you might write a character who was forever noticing a total absence of pain in his left knee, and indeed most characters do have left knees that don't hurt, but the rest of us out here in Reader-Land wouldn't see why that mattered). This tends to create Mary Sues who are condescending, passive-aggressively spiteful, impossibly nice, or all of that at once, and they really stick out like sore thumbs.
The remedy, if you're having a sinking feeling about that Mary Sue character, is in steps.
One, find something to be good at. Bruce Sterling did a nice job of explaining it in Heavy Weather: everyone needs something they can hack, i.e. make do whatever they want it to do. Bake the best muffins, fix old typewriters, clean the bathroom faster and cleaner than anyone else, but get something you excel at. (Big warning: some activities, like snarking, eating, hurting feelings, or disgusting people, are impossible to excel at meaningfully).
Two, find out what that feels like inside.
Three, think about your Mary Sue's reality. Being really good at anything comes in very different and highly specific flavors, not in a broad brush haze of happy self-satisfaction. Narrow it down to his/her specific experience of goodness.
If Madame Parfait knows that the moment someone pulls a neural disrupter she'll have hers out and firing before the thug that drew on her has his all the way out of the holster, then she's probably at home in the spaceport bar and the presence of rough types is about as meaningful as a couple of yellow jackets at a picnic; she has to watch them but not fear them. If Sir Ultimate has killed four dragons but come close to being killed twice, then when he goes in for that special place in the pectoral scales, he's terrified but he's on top of it -- and that's not at all the same thing as Madame Parfait. If everyone sucks up to her because she's the beautiful daughter of a billionaire with a penchant for petty vengeance, Lucienne Ihateher isn't surprised or arrogant about the attention but she does expect everything to go her way; depending on many things, she may see her "success" as bogus (and be trying to avoid being reminded of it) or think she hit a triple because was born on third base, but one way or another, the success is just there, like her height or her thumb. Winning comes in more flavors than losing (and it's not just the absence of losing, any more than vanilla is the absence of chocolate).
One last thought about this, peripheral to the Mary Sue problem: there is sometimes something curiously flat and odd in tone when people write about being good at something that they are actually really good at. I'm fairly indifferent at some of the things I describe very well, and quite good at some things I never feel like I get right, and I have talked about this with half a dozen writers who have similar experiences. I have no idea why this is.
The key, though, is that to describe success (and thus characterize Mary Sue) accurately in anything, you have to have experienced it, for real or vicariously, at something; that's how you find the specificity that makes it real.
Possible hidden cause of the Mary Sue problem #5: s/he is actually from a too-small conventional world, rather than from your fictional one.
If Countess Mary Sue is a medieval aristocratic woman in a Europe of feuding petty lords, she is very apt to have virtues like pragmaticism (she knows what has to be done if the castle is attacked while the lord is away, and how much has to be in the silo and the smokehouse to get them through a winter siege); fierce personal loyalty (she might wait a decade till she can avenge a kinsman's murder); certain kinds of Christian charity and pity (and in her world, pity is an important virtue).
She is less likely to be gay-positive or broadly in favor of learning, so if she needs to be for the story you want to tell, you'll need to deal with why and how she's that way. And not having encountered the kind of racism that begins with the modern age, she really isn't likely to be anti-racist as we know it.
This doesn't mean she can't be; perhaps a portal to the Unseen Realm opens in the castle's chapel, and she accidentally invents most of the modern world among the gnomes and fairies before what has happened teaches her some valuable lessons while giving the reader an insight into what our culture became instead of what it might have been. But the farther the virtue is from Mary Sue's culture, the more the story must be about how she happened to have it; you can't just tack it on like lace curtains on a missile launcher. Remedy: Take the culturally improbable perfections and either cut them or make them what the story's about.
*I've always wondered if it was just that female fanfic writers were more willing to let other people know they wrote fanfic, in those long-ago days when it was a subject of shaming, which happened to coincide with a period in English-language culture when women found it easier to have friends than men did.
**Some fan writers referred to such male Mary Sues as Marty Stus, but that never seemed to me like a name a guy would give himself. I would bet that if you are a guy and your name is Martin Stuart Somethingorother, you do not tolerate people calling you "Marty Stu" and I, for one, would not fault you if you popped the presumptuous son of a bitch a good one in the nose.
***If you want to write something good, difficult problems are a good thing. It's my belief that most great stories solve, successfully or not, some almost-insoluble problem in fiction. Really sympathetic villians are hard, but that's why MacBeth, The Godfather, and Paradise Lost have such staying power. "an overly perceptive privileged kid sees something awful and is helpless to fix or stop it" sounds like a snoozefest but it's the basis of The Catcher in the Rye, The Great Gatsby, To Kill a Mockingbird, and (a personal favorite many of you will disagree with) Less than Zero. So if you suddenly realize you have tried to write something that is difficult or impossible, and it was not just pure folly, then dig in and bust a few shovels and your back. You are on the ground where fictional gold is found, if you are ever going to find it.
****For commercial purposes, sooner is better.