but the way Leia, Han, and Chewie treat her goes beyond that. I think (or hope) it's meant to indicate that they knew her or her family before, not that she's the most wonderful girl EVER! We shall see.
But the essence of a Mary-Sue is that they are a projection of a writer's unrealistic fantasies, they are what someone would like to BE, and by that definition Luke Skywalker is a Mary-Sue. He's the kid from Modestoine that wants to get out of there and do amazing things, and he develops superpowers with minimal training and saves the day. IMHO he's a very well-done Sue or Stu. Having a touch of a Sue isn't necessarily a bad thing, a really well done Sue or Stu can tap into a lot of people's fantasies about what they want to be, like James Bond.
Luke has a few Mary sueish elements in his character but rey is as Mary Sue as it gets. The comparison isn't even close