Just an note about a discussion I had a year ago (last fall semester, actually) in my MFA program about Mary Sues. We had a visiting writer speaking to us and she made a comment that, since everyone hates Mary Sues, perhaps we ought to use them a kind of archetypical anti-character, one that our readers will immediately dislike. She was being facetious, but I work with a woman who falls into that category: she's blond, pretty, skinny as a rail, smart and personable, and all those in authority just love her! What would we do without her! Eggghh. I've killed her at least twice in writing exercises.
I've written on forums for a few years, especially the FF.net forum, and I've been the phrase Mary Sue tossed out where it clearly didn't apply. In fact, I've seen reviewers use the phrase to suggest the character is like the writer - but unless you KNOW the writer, how do really know the character is like them? The Stephen King example works because everyone knows a lot about King, but we don't really know what's on the other side of an avatar on a forum unless we know that person or they've revealed something.
Another element is that beginning writers are told to (and should) write about what they know. They know themselves, so that's not a bad place to start, is it?
I think some reviewers apply the phrase Mary Sue to any story they simply don't like. The phrase often get used when there is insufficient explanation for character motivation. A Mary Sue is character equivalent of the deus ex machina problem: everything goes her way, and there is some utterly ridiculous solution to all her problems. Think Downton Abbey.
Bad motivation is different. Recently, I read about a year's worth of confession magazines as part of a literary assignment (don't ask). These are FILLED with horrid motivations: you know, mothers who, for no explanations, suddenly have sex with their daughter's husbands and crap like that. As others have said, that's not a Mary Sue, that's just bad writing.
I don't hate Mary Sues. A number of people in my MFA program seem to hate them, and seem to enjoy telling people how much they hate them. I'm kind of meh. If I like the story enough, I keep reading/watching. After all, Anna is my favorite character on Downton Abbey and she's definitely a Mary Sue.