Hmm...maybe it's a teen thing. My friends and I take it to be less of "living through fiction" than as a ridiculously perfect character. And then there are those extremely flawed characters who, through no fault of their own, become extremely well-liked. I've never seen this happen, but I can totally imagine it, maybe in a satire. And the term is too subjective (Morpheus the Sandman a Mary Sue? You gotta be kidding me...) along with the Mary Sue Litmus tests. I think they're intended for aspiring teen writers, trying to get them to let go of their fears of writing flawed characters. Or something like that, anyway.

"And, too, I'm left to wander what the polar opposite of the Mary Sue character is, because, in truth, that must be what I write, most of the time."
The anti-Mary Sue? The Bloody Mary Sue?