A character or that is so perfect it’s impossible to relate, making the character boring and often resented.
A Mary Sue (sometimes just Sue), in literary criticism and particularly in fan fiction, is a fictional character with overly idealized and hackneyed mannerisms, lacking noteworthy flaws, and primarily functioning as wish-fulfillment fantasies for their authors or readers. Perhaps the single underlying feature of all characters described as "Mary Sues" is that they are too ostentatious for the audience's taste, or that the author seems to favor the character too highly. The author may seem to push how exceptional and wonderful the "Mary Sue" character is on his or her audience, sometimes leading the audience to dislike or even resent the character fairly quickly; such a character could be described as an "author's pet".
- from http://wowrp.wikia.com
The term "Mary Sue" comes from the name of a character in "A Trekkie's Tale" - a parody by Paula Smith. In this story, Mary Sue is a fifteen year old Star Trek Lieutenant, who's beautiful, smart and highly skilled in almost every way.
Read more here http://en.wikipedia.org/wiki/Mary_Sue
Especially in WoW (and other roleplaying games), it can be difficult to avoid creating such a character, as the fantasy setting makes it possible to be anyone you want to be. Most people naturally want to make their ideal character, and while it's possible to pull off, it's difficult not to cross the line into 'Mary Sue teritory'.
The quick tip to avoid creating a 'Mary Sue' is to give them some flaws, and not always let them win.