1. They don't have flaws. Flaws are the first thing that should be explored in character, at least after the first flashes of inspiration have passed and you start moving into more concrete turf. People say Mary Sues are defined by lack of flaws; the flip side insists that giving your character "issues" won't make a character any less of a Mary Sue. Now, my definition of a Mary Sue doesn't involve flaws or the lack thereof; a Mary Sue, to me, is defined by the disproportionate amount of attention given to a Sue by the plot and the priorities of other characters. So, yes, it possible for a Mary Sue to have legit flaws and still be a Mary Sue. However, the difference between "issues" and a character flaw is what role they play in the character's development. If a character is a cutter, and constantly whines about their lousy, black-shadowed lives, and that's all they do: that is a character with issues. If a character is a whiny cutter whose self-hatred drives them to find a better way and moves them to grow and change: that is a character flaw. The flaw is self-hatred, not the cutting. Flaws cut down their bearers or are overcome by them; the middle ground is flaws make the character suffer repercussions.  If our cutter sits around telling people they cut because they hate themselves, and every single person around them goes "You poor thing, have some chocolate", that is not what you want. If someone is a jerkasshole because they hate themselves, and someone justifiably smacks them around or tells them to shut the hell up, the jerkasshole has suffered repercussions for his self-pity. Characters worth knowing and rooting for are not victims; i.e., not people who sulk and feel sorry for themselves; remember that the hero element of any character is watching them strive for something better. If any of you have been following my Kit Farson stuff, one of Kit's big flaws is fear. He's afraid of everything, especially other people, and he's either shy and anti-social because of it, or angry to cover up his fear. Kit struggles to overcome his fear and do the right thing, but his fear earns him repercussions: he's bullied because he's shy, he gets in fights because he gets angry, and he's very often left all alone because he's anti-social. This flaw keeps Kit from things he might really desire in his heart of hearts, like respect and true friendship, but he is actively trying to get past his flaw in order to get those things.

2. They are narcissists. This is a big one, especially here on DA. I suspect it has to do with so many folks here on DA being teens or tweens, learning for the first time that you are not the center of the universe, not as special as you thought, and that being a good person sometimes means you have to put other people first (don't worry, some people never figure that last one out.) I certainly had characters when I was that age that were free of societal cages; they could insult people, and walk all over authority figures, and they were empowered like I wasn't. But, of course, to the rest of us, this kind of behavior comes across the same as when you see a spoiled teenager. This gets worse when you have characters with things like a prophecy and hero designation and "speshul" markers like telepathic wolf companions and such. When you combine "speshul" with jerk, we grind our teeth. None of us our special, and if we do become special, it's rarely because of providence. Lots of characters fail because they are jerks inside and out; we don't want to get to know them. They demand the world be a certain way instead of shouldering the burden to change it themselves; they push others out of the way to further their own goals instead of extending a hand; and are very often cruelly insulting while demanding respect and empathy. If your character is anti-social and crazy, they better get their asses roundly kicked at some point (unless they're a catalyst, and then you'd better A) know what that is and B) not have them as a protagonist.) And take the time to at least balance them out with endearing traits only the reader can see. Moral absolutism is another aspect of narcissism, when the character equates their point of view with "right". This happens a lot with Mary Sues, and can really get scary as hell. As Billy Joel said "The only people I fear are those who never have doubts." Show me someone who is so right they're willing to kill over it, and I'll show you a dictator-in-the-making.

3. They never learn. This is a favorite of people who create the "Screw the rules!" characters. The crazy people who do whatever they want, live by their own rules, and aren't bound by petty things like "empathy" and "responsibility". You're supposed to feel sorry for them if they do suffer a setback, like getting punched in the face for being outrageous, but don't ask them to change. They are who they are, man! While there is a strange reinforcement of Heroic Sociopathy, I guess because people like the idea of not ever having to compromise, it really gets annoying when their vantage point is portrayed as justified. And when you combine this with the "crazy for the sake of being crazy" motivation, it's god-awful. And way more popular than it should be. If you have ever roleplayed, I guarantee you have encountered this character type before. If you're roleplaying to explore character and growth, and you're playing this kind of character: stop. This type of character is called a catalyst in writing, a character that forces change out of other characters, but never changes themselves. While this is a legitimate force to have in storytelling, I really hate seeing people insist that their never-do-anything-wrong character is really a deep and nuanced protagonist, someone who has to change. How can they change if they're never aware of doing anything wrong?  If they don't care that they have? If they react to repercussions by delivering one of their own that's ten times worse? A character that never learns is never wrong, and never changes. It's that simple. The character who insists they're doing right and doesn't care who gets hurt: that's a villain.

4. The author is afraid to humble the character. This happens a lot whenever Mary Sues are around, or at least it's a sure sign that a character has something like Plot Armor or an Author Tract protecting it. The character never fails in their endeavors, never doubts, never fears, and most importantly, is never rebuffed, set back, or punished by external forces. Now, a Mary Sue will simply tromp right through such things, but many authors will have the character quietly dodge the obstacle or never have the character confront it at all, and still claim that their character really went through something life-changing. It's having cake and eating it, too, without the satisfaction of sacrifice on the characters part. Rather like Bella Swan never having to confront the dark side of vampirism: she's never forbidden from it by her father, her new state doesn't alienate her from her family, nothing is said about her setting herself up to watch her Mom and Pop shrivel and die, and definitely no one ever says anything about the werewolf child-grooming her kid. Repercussions are a huge part of character growth, but the instinct of most young writers is to shy away from it.  (And I know why, because most are still teens living at home, dealing with the frustration of having your own ideas without the freedom to act on them. So you write about characters who can go where you can't and don't have to deal with the frustration you do.)  Especially if you are roleplaying in order to explore character: if you are not doing this, you're missing the point.

5. All their change happened before the story. This one was actually pointed out to me by an old roleplay buddy of mine while we were discussing character. How many character profiles have you read or seen where so-and-so is an assassin, who suffered years of Training From Hell, and killed a dragon, and lost his one true love, and got really bitter and anti-social about it. But they're the main character in a story that mysteriously, somehow, can't be written. It never seems to occur to folks that that character's past would be a novel in its own right. Not only that, it seems to be the habit to just put stuff into a backstory for the hell of it. "And then, I beat a whole bunch of pirates!" "What do pirates have to do with our story? We're in the middle of the Old West." "Cause … cause killing pirates is cool." Character can start with a simple concept, but don't get so dazzled by it that you forget the journey. Most stories are about watching characters suffer; a favorite saying among my writing friends is "I'd never want to read about anyone who wasn't at the end of their rope." If you have a demure smug jerkass whose totally awesome at fighting and kills stuff all the time and goes home to sleep with his true lurve-4ever girlfriend … that's not a hero I'm interested in. And don't think having him go home and cry about all the people he killed makes him "deep"; it doesn't. Character is nuance, and a lot of that nuance is in the journey.