I think of it as wish-fulfilling insertion, too, but TNH's post says she's been using "Mary Sue" as shorthand for J. Perfect Character in submissions. I can see it might be useful for that, but just the same it's making a term less specific, and I never like that much. (Mary Sue still is and should be distinct from self-insert, which can actually be good, at least in theory. We just need a third term.)