I think the problem with Rey stems from a lack of real character, as well as true obstacles. I mean, obviously the force enables certain people who are "force sensitive" (since I guess we're tossing the concept of the Jedi out the window now), so of course she's gonna tear shit up. But I think the issue with her is we don't really see her undergo the hero's journey of overcoming real weaknesses, obstacles, conflict (unless you count her brief infatuation w/ Kylo and trying to turn him for some reason), and thus she just feels very wooden to me.
It also almost feels as if Disney wanted to make her as "neutral" and without a real personality as possible so that people can place themselves in her shoes and feel like they're her, but that's not what making a good, interesting protagonist is about IMO, it's about relating to them on a personal level. I just don't really get that sense with Rey.
Whether you interpret that as being "Mary Sue" is up to you to decide because the definition seems somewhat fluid and relative, but all I know is that's the way I feel about her character.
"We hold these truths to be self-evident - all men and women created by the, go-you know.. you know the thing!" - Joe Biden
