One very important thing I keep in mind is that someone's wish-fulfilling fantasy -- the Mary Sue -- is beloved by no one else but the author. The more you are infatuated with your own character, chances are, the less everyone else will like the character.
I think this one comes back to keeping a proper sense of distance between you and your character. If the character is nothing but a projection of one's own wish fulfilment, it is certainly not likely to appeal to others. Dreams are very personal things, after all.
