A Mary Sue isn't always a perfect character, either. Sometimes he/she is the uber-smart hacker, the cool rebel, the superhero's sidekick, the comedian whose jokes never fail to cheer up the heroine when she's feeling blue, the evil villain/ness who the heroes have to struggle to beat despite numerous plot holes, etc. It is usually whatever the author wishes he/she could be, though.
As for self-projecting, if you don't put a little bit of yourself into your characters, they're going to fall flat and seem two-dimensional. Say you have a dog, and you want your character to have the same breed. That's fine up to a point. If you want to be the best dog trainer ever, and your character turns out to be that, then you've got a Sue. The trick is to not put too much of yourself in and to know when you've got too much (or to listen to people when they tell you that you've got a Sue and not get mad at them).
