You know something that straight away makes an OC look ridiculously Mary Sueish? Well, aside from all the usual stuff like the silver hair, the red eyes, the curves in all the right places, the extensive wardrobe described in snore-inducing detail and the tendency for any more than moderately attractive canon character in the vicinity to be struck dumb and stupid by her awe-inspiring beauty, that is?

Over-endowing them with special powers.

No, I can't think of a neater way to phrase this, but it's the only way I can think of to put it that doesn't make the sentence look utterly nonsensical. So over-endowing them with special powers it us, at least until someone out there can think of a better way to put this.

Anyway. We've all seen them, haven't we? We've all seen those cute teenage Sues that are the bestest of the best at everything they try their hand at. Anything canon can do, they can do better. And boy, it gets aggravating. It gets aggravating real fast. They out-fight the warriors, out-smart the intellectuals, out-sparkly the mages. They're the bestest at the best with computers, guns, swords and extra-sensory perception and they cook, too... and so it continues until you the reader want to throw them out the window just so that the regulars actually have something to do for once. What I mean is it's annoying. Like, really annoying.

Not, of course, that there's anything wrong with an OC displaying unusual abilities: I just wish more people out there knew when to call the whole thing off...

Okay - fandom-specific examples here, but really this could apply to any fandom out there in which characters possess any kind of unusual talent or skill, or even usual talents. Feel free to sub in your own names, skills and what have you - this hasn't really got anything to do with my fandom, I'm just using it as an example because I can.

Anyway. In my own fandom (Weiss Kreuz, for my sins), a few characters - a very limited number - have psychic powers. These powers, the canon points out on a number of occasions, are also strictly limited. For example, a precognitive character states quite categorically that his gift allows him to see 'only a little bit of the future'. One of his teammates, a telepath, mentions to one of his companions that he sometimes finds his own powers overwhelming: he doesn't enter his targets' minds, but experiences their thoughts alongside his own. Sometimes, he points out, this can become somewhat confusing. These people have psychic powers, but they are not all-powerful, and there are implications to their possessing them. Hell, the fact that they possess psychic powers has brought them no end of difficulties.

The cliché Weiss Kreuz Mary Sue, however, often has more than one psychic power. She's telepathic and telekinetic and she's amazing at both. And she's never had any problems because of it - she's just a normal girl with amazing talents who just so happens to be a trained killer but is still more worried about getting a cute date for the prom than whether or not she's actually going to be alive to see it. But I'm getting off the point here.

The point is that Mary Sue pays no attention to canon limitations on her speshul powers.

Which is where it gets annoying even before one takes into account that this supposedly 'normal' girl is running rings round trained killers [battle-hardened soldiers/experienced ninjas/Johnny Depp in killer eye make-up]. The characters who, in canon, boast the powers she's claiming are either much weaker than her - not because they themselves are weak but because she's so overwhelmingly strong, and far more so than any of her canon peers - or their own powers come complete with a downside which she's never had to experience, or both.

But the abuse of special abilities is the symptom, not the disease - just as it's perfectly possible to write about a beautiful OC without writing about a Mary Sue as a consequence, it's equally so to write about a powerful one. All you've got to remember is to have a bit of restraint.

There's no problem with giving OCs special powers, should the universe be right for it - magical powers or empathy alone does not make someone a Sue. In and of themselves, the abilities are not the problem - they're just a tool, a means to an end. It's perfectly possible to create an OC who has a special power of specialness without making them look like a massive great honkin' Mary Sue for it. What it comes down to is respecting the norms of the universe you're inserting that character into, rather than gleefully exceeding them because you can and it makes your character look more important.

If your fandom doesn't boast all-powerful characters, parachuting in an all-powerful OC looks suspect. It looks equally suspect, in scenarios where such talents aren't even remotely common and those who have them tend to get no end of trouble for it, if a random teenage schoolgirl with a perfectly normal (or even Mary Sue normal) past and family background is suddenly revealed to be a highly-powered telekinetic empath. And it looks just as stupid, if not more so, when a child or young adult OC is not only running rings around the best of their peers but outclassing grown men and women with years of experience, who have been expert in their field far longer than the OC has been alive.

In short: be reasonable.

There is nothing wrong with your OC being talented, but she shouldn't be so talented she makes the rest of the cast look like clueless five year olds - that's where the problems start. If you the ficcer respect the limits that canon has set out, your characters will look a lot less out of place. Simple enough, isn't it?