[Anareth]

I think it's when the "unreality" of the character is taken to absurd extremes is when it strays into Sue/Stu territory. But I agree that's not itself a defining characteristic.

Also, you have to be careful with "I gave him/her flaws!" as a defense. Especially as the most common attempt to dodge the Sue/Stu label is to give them flaws that aren't. For a professional example, look no further than Bella Swan (for chrissake, her name means "Beautiful Swan"--I C WAT U DID THERE), with her "I'm too pale, too thin, too gawky, I have dark hair, and I'm SO clumsy." Because being a pale thin brunette is such a social curse. And the clumsy quickly passes into absurd territory. She's still a hopeless Mary Sue and author avatar to boot.