“If a heroine is especially good at anything, even if it’s just one thing and she’s spent years working on that skill, she may get called a Mary Sue. If she’s a relatively well-balanced individual without PTSD and a string of broken relationships and powerful enemies hunting her from the get-go, she may get called a Mary Sue. If there’s anything about her life that you would enjoy in your own, she may get called a Mary Sue. In short, if she’s the protagonist of the story and there’s anything admirable about her, then to some readers, she’s a Mary Sue.”
Marie Brennan, “Who Mary Sue Isn’t” (via nonisland)
This is a super summation of an ongoing problem: with a good suggestion at the end regarding what to do about it.
Yup. People like to use the term Mary Sue for any female character they don’t like, much like they like to use the term Deux Ex Machina and/or Plot Hole for any developments they personally dislike. It’s a trend: subverting legitimate literary critical terms for petty, minor and unrelated grievances.