My Little Pony LVI:S Has Left the Playground!

[Aotrs Commander]

Well, I suppose I've always associated the notion of Mary Sue with the desire of the author to live vicariously through the character. The thought of one of the writers on the show wanting to actually, ehem, be Twilight Sparkle seemed kind of silly to me, you know?

Sue are often self-inserts in fanfics (principally where the term originates), but the use of the term has broadened to more generally, characters that are way-overpowered and/or taking the focus away from the other characters, or have other gratuitous flaws. (Wesley Crusher (From ST:TNG) is a commonly cited (perhaps a but unfairly) example.)

As always, wiki is your friend.

(And, before anyone gets pedantic and we get down to pointlessly arguing semantics, as Lix said, sueish, e.g. having some traits similar to a sue.)