I think the cases that have been made that Rey is a Mary Sue forget that if what they considered the same set of traits were comingled in a male lead character played by a Tom Cruise that we'd be having different comments. Not being a woman myself I do wonder how many eye-rolls there have been at male heroic leads in films who were as "perfect" as some have said Daisy Ridley's Rey is. If you want a Gary Stue, just consider the extent to which some Star Wars fans claim Palpatine controlled everything from Episodes 1-6 when he says "Everything that has transpired has done so according to my design." Instead of taking this in the context of conventions of action adventure films of the Reagan era (i.e. villains talk smack and talk big as if they are more awesomely competent than they actually are) some guys insist that EVERYTHING that transpired from the start of Episode 1 to the climax of Episode 6 was engineered in some fashion by the Emperor.
When fans of the franchise are actually making serious attempts at anything like "that" claim, it's impossible to say that Rey is a Mary Sue within the Star Wars universe, not when you see what some fans actually claim Yoda and Palpatine can do. Rey is fairly "normal" perfect for a comparable male heroic lead in an action film.
Considering how many action movies we have it's not "that" conventional yet to have a female heroic lead in an action adventure film who is not a trophy or arm candy. Bear in mind that Star Wars still hews more toward the all ages audience rather than the more PG-13 dystopias of Hunger Games.
So how many heroic action leads are female in films that are suitable for all ages that may have a woman who is considered pretty but is also not sexualized and does not trade on sexuality in some fashion?
That list could be ... well, if you stop and think about the history of the action genre Rey might just be the only one. Well, okay ... maybe Twilight Sparkle but that's a different kind of genre.
So Rey seems too perfect because she's a competent scavenger, interested in machines and figures out how to fly a certain spaceship with enough skill that she gets props from Han Solo. Let's bear in mind that back in Episode 4 Luke says a certain shot is, in a word, kinda easy. He didn't say this because of Force skillz but because he lived on a litter box planet full of sand and had nothing much else to do except test out marksmanship. The idea that Rey can't be an inventive mechanic who can improvise solution on legendary freighters doesn't seem that far-fetched in the Star Wars universe. A girl who has a burgeoning interest in mechanical things, science, and at least some interest in staff-fighting should find Rey a delight to watch on screen. I'd never heard of Daisy Ridley before Episode 7 but now that I've seen Episode 7, well, she's definitely a star now.
Rey is what Korra from Legend of Korra should have been if she was well-acted and written by writers who had the slightest clue how to write for women characters, something I was swiftly convinced by seasons 1 and 2 of Legend of Korra the key writers either no longer knew how to do or maybe never knew how to do to begin with.
Remember back when Megan Fox was on press promotional duty and explaining how she was playing a "strong female character"? No? Well Daisy Ridley's Rey, in terms of an action/adventure film heroine, is entering the fray in competition against that Megan Fox character. Is this really a contest which of the two characters you would present to your daughters or nieces as the one most worthy of emulation? This seems like a no-brainer to me and I'm not even a parent. If Rey and Finn seem like retreads of characters we've already grown to love compare them to the characters played by Shia Labeouf and Megan Fox in the Bay-formers franchise and I think we could find it in our hearts to go easy on the Ridley and Boyega characters. There's a difference between casting a pretty woman from England as an action/adventure movie star and casting a ... Victoria's Secret model to play Sam Witwicky's girlfriend in the third Bay-formers film.