Mary-Sue is a slang term used in fan-fiction and role-plays. It's when the writer, usually female, writes a version of herself into the story. Only, this self is incredibly beautiful, intelligent, good at everything, and a main character.
Every teenage girl (and possibly, boy) goes through that stage. I did, but fortunately, I was over it before I knew about the internet. It's great fun for the author, but quite annoying to everyone else, who does not identify with this fantastic, fantasy character.
I've said, in a previous post, that Harry Potter, Twilight, and Buffy the Vampire Slayer seem, to me, very Sue-ish. They are so successful because every teenager (and many adults) who read the books want to be the main character. They identify with them, and so, support them. And, as with any Mary-Sue, to anyone who does not identify with the main character, this habit is extremely annoying.
However, one adult book did manage to carry this off - Marianne Stilling's Damsel in this Dress. In many ways, it's an adult Mary-Sue, but, instead of being annoying, it succeeds in being hilarious. I don't know how she's done it, but it manages to be funny, reminiscent of my own mildly embarassing teenage fantasies, and highly enjoyable. Well done.
