As far as her being a Mary Sue … what flaws or limitations did she have? And even if you give her flaws and limitations, but you come up with the excuse that she has some innate ability to overcome those limitations at the whim of the writer, then what is she exactly? This all gets back to how poorly thought out the plot was. Instead of creating a genuine, original story, we had JJ and Kathleen sitting in a room listening to some Disney execs tell them that THIS, and THIS, and THAT, and THAT, and these other 50 things all have to be in the movie to make the most money. That was a recipe for disaster. The plot wound up as an incoherent mess, and that’s really the only criticism I have of Rey’s character. She was basically given “liquid luck” (Harry Potter reference) for any scene in the movie the plot could use it.
From a post I made earlier: “She’s not unflawed. She’s got interesting trust and family issues that are obviously due to what happened to her as a kid and cause her to reject Solo’s offer. It’s a nice arc that’s resolved when Finn and co. come back to save her and she realizes they actually care about her. Of course she also fears her past and the unknown truths about herself which manifest when she touches the lightsaber. She doesn’t just become a full-blown, lightsaber-toting hero right away.”
Her weakness is not external but internal, which in my mind is a pretty good way of showing her vulnerability and humanity.