At his inimitable blog, author John C. Wright has an interesting blog post (which is mostly a quote of one of his readers’ comments) about what makes a Mary Sue. The key insight is that the defining characteristic of a Mary Sue is not that she is super-awesome. It’s that she’s super-awesome but we’re supposed to treat her as a young, innocent ingenue:
Rey is great at everything she does. The reasoning behind that may be justified…but that is not the issue.
The issue is that Abrams clearly expects us to think of her as a sort of female Luke Skywalker. Except Luke was nothing like that! We are asked to accept that she is a natural born pilot, better mechanic than Han Solo, better natural Jedi than Luke was at the same point of his training, and a natural swordsman…but we’re ALSO supposed to think of her as a plucky orphan farmgirl.
I think that’s right. A 35 year old queen who is beautiful, intelligent, a skilled warrior and a crafty statesman wouldn’t be a Mary Sue if she’s presented as someone with a past who’s used her 35 years to good effect—if she’s someone who’s already been on the hero’s journey and come out of it having learned some lessons. The real problem comes in when she’s all of those things and only 16 years old.
The commentor which Mr. Wright is quoting calls it fundamental dishonesty, and while I think that he’s right, I’d prefer to call it a fundamental contradiction in the character. What really makes Mary a Mary Sue is when she’s got all of the benefits of experience without having any of the experience. I think that it really comes down to sympathy.
Growing up is rough so (sane) human beings have sympathy for people who are still doing it. We are willing to tolerate all sorts of mistakes in those who are young and inexperienced which we will never tolerate in the old and experienced. (This is why it’s so important to not waste one’s youth and to learn how to be competent while people will still be forgiving of your mistakes.) The upshot is that young protagonists are much easier to write—the audience will naturally be sympathetic with them. The author’s mistakes will get much of the forgiveness that the character gets since the author’s mistakes often are also the characters’ mistakes.
The other thing is that this makes character development a snap. Children don’t know anything and make (nearly) all the mistakes one can make, so giving them something to improve about is trivial. Because of the instinctual forgiveness given to children, they don’t cease to be sympathetic merely because they start out awful, either.
This creates a temptation on the part of the author to make his character younger than he should be because it’s easier to write. A competent adult is much harder to make sympathetic, especially if he has a character arc. It’s easy enough to give him a character arc if you make him start off as a bastard who deserves to be shot. (This is the reason why the Loveable Rogue™ is to popular, by the way—just be careful to say that he’s a rogue rather than show it or the sympathy goes away. Show don’t tell does not apply to flaws in characters you want the reader to like!)
What’s really hard is writing a competent adult with a character arc who starts off as a decent human being. The reason this is much harder, of course, is that the writer has to be better than minimally decent. Because one can’t give what one doesn’t have, one can’t fake wisdom. And the character arcs of decent adults are all about growing in wisdom. A child has the (easy from an adult’s perspective) task of becoming a minimally competent adult. A minimally competent adult has the task of becoming a wise old mentor. Children generally succeed; adults often fail. For that reason, nearly anyone can write a coming-of-age story. It takes a real man to write a story about someone who already came of age.
A Mary Sue is the attempt to have it both ways—to write a coming-of-age story about a competent adult.
Chris Lansdown 
I like your points about showing and not telling and how already decent adults can have difficult character arcs to write. I still disagree with John about the meaning of a Mary Sue.
Mary Sues are idealized authorial stand-ins. The disingenuousness and effortless ability are, I would say, inevitable though not necessarily ubiquitous consequences of this self-idealization. But a Mary Sue would still be a Mary Sue should these other factors not be present.
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I think that a “Mary Sue” has come two mean two different (though similar) things. One being the omni-competent ingenue, the other an author self-insert. They can be the same, but don’t have to be. I think that the latter—the author self-insert—is really a problem when they exist in the story to receive rather than to give. A character who passively receives good things isn’t interesting (except possibly in pornography, I suppose). A character who works and suffers will be interesting even if they’re the author but 6″ taller and with 22″ biceps.
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The problem with the “stand-in” is that, even though the term started there, it means you have to revise your opinion of whether a character is a Mary Sue based on making discoveries about the author. This is critically unsound.
The big problem is that the term “Mary Sue” means something bad and so it gets abused.
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I think the connection between a “Mary Sue” and an author self-insert is accidental, although extremely common. In the case of Rey, I believe the primary driver of the defects was not any self-insertion, but the new “wamen have no flaws” trope that Hollywood has decided is good because feminist, nevermind that it utterly ruins a story. This really is one of the problems of our culture, the conflict between (superficial) diversity as the left understands it and good storytelling. To take Marvel comics for another example, they have added many new black characters, but all the new black characters and all the existing black characters have melted into essentially “generic black woman” and “generic black man,” “soulful saint” and “Gordon Goodbrother” to use Diversity & Comic’s terms. There is a LONG storytelling tradition of the “black badass,” from Stagger Lee, to Bad Bad Leroy Brown, to Shaft, and so on. Characters like Luke Cage and Blade used to be very much the “black badass” trope—no more. The black men of Marvel have all been gelded. Why? Diversity! You have lots of black faces, but no black characters with FLAWS, and therefore no black characters who matter, or have good stories. Contrast a character like Killmonger from the Black Panther movie.
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What gets me is that it’s not like all creators are completely untalented. It’s that these utterly ridiculous and fallacious beliefs about politics, and human nature in general, dictate what they create.
The end result is boredom and bad art.
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“A Mary Sue is the attempt to have it both ways—to write a coming-of-age story about a competent adult.”
That’s one way to do it. There are others.
When not the story but the universe revolves about your character, it’s a Mary Sue.
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