If I remember right, a Mary-Sue is a character that is so perfect that you want to kill him/her just because of it. In other words: the character is capable of 'anything', all the cast are his/her friends, never gets hurt, and a few other things I forget at the moment. (there was a mary sue test...I'll need to see if I can find it...)
If you remember Spider Silversnow, he was a 'borderline' Mary-sue.
Edit: Here's an quote about Mary-Sue's from
-this- site:
"You already know Mary Sue. Mary Sue is the perky, bright, helpful sixteen-year-old ensign who beams about the ship. Everyone on the ship likes Mary Sue, because Mary Sue is good at everything. Mary Sue is an engineer, a doctor in training, a good leader, an excellent cook, and is usually a beautiful singer. Mary Sue often has mental powers that may manifest themselves as telepathy, precognition, or magic. If Mary Sue is very young, she is often the offspring of one or two already established characters. If she's a little older, she will probably end up sleeping with the author's favorite character. Her name is often the author's name, be it a net.name, a favored nickname, or the author's middle name (this is seen in the most famous Mary Sue of all time, Wesley Crusher, who was named after Trek creator Eugene Wesley Roddenbery). By the end of the story, Mary Sue will be in bed with the desired character, will have beamed away amid cheers from all the regulars, or will be dead, usually accompanied by heavy mourning from the cast. The reader, on the other hand, will be celebrating. "