I found a couple of essays on Mary Sue's that I skimmed. They're very long, so I didn't read everything in them:
http://en.wikipedia.org/wiki/Mary_Suehttp://www.merrycoz.org/papers/MARYSUE.HTM What I wondered about is this; if an author places themselves into a novel, but is careful to use character traits good and bad which apply to them, will the character always be classed as a Mary Sue, or is that only a case where the number of positive traits outnumber the bad?