TV Tropes goes into some detail as to why the term has lost its meaning, but the gist of it is that it was originally intended to describe perfect female avatars of authors writing fanfiction, since at that time many online fanfictions were written by women. The term could also be used for perfect male author avatars (Gary Stu), but this term never saw wide usage since there already existed a term for this character: Action Hero (James Bond, John Matrix)
LOL! This is a ridiculous attempt at equivalence. And that’s the last I’m saying on this subject.
That’s a shame, because I really don’t know what is ridiculous about comparing male and female versions of power fantasy characters. The fact that one version has expanded into a genre and established itself within popular culture whereas the other has acquired a pejorative usage doesn’t negate the fact that action heroes are often Gary Stus. Not all are, but quite a few. To take another comment on the BMD article, Vin Diesel is a Gary Stu in practically everything he makes. On the literature side, the Name of the Wind, a book that my brother is enamored with, centers around a classic Gary Stu. And I enjoy Vin Diesel’s movies (for the most part) and The Name of the Wind. My point is, I don’t think people should criticize Mary Sue or Gary Stu characters simply for being author avatar power fantasies, but I do think it’s enlightening to realize when a character is essentially that.
Also, as I’ve pointed out already, I DON’T THINK REY IS A MARY SUE, and even if she was, I would have no problem with that if her character appeared in an original work. I totally understand why people get defensive when the term gets thrown around, but female power fantasies are a genre, and I don’t think people should judge a genre to be wholly without merit. The problem is that due to insufficient backstory and unearned character moments, she feels like a Mary Sue in Star Wars, which has never been a power fantasy (despite what some people say). It’s a problem with many small elements of plot which are omitted throughout the film, culminating in a character that feels less than coherent.