I'm kind of surprised at the amount of people that hoard some disdain towards Rey as a character, even if it isn't necessarily the majority. She was what, in my opinion, held the movie together and truly made the movie "feel" like a Star Wars movie.
The original Star Wars, especially Star Wars and Empire Strikes Back had a very specific, serious, heavy feel. There were light hearted moments to help break this up, especially driven by C3PO and R2D2, but throughout all the films there was a sense of longing, loss, melancholy and even desperation that seemed to drive the story forward. Because of this, Star Wars is real, gritty, and often not pretty: Lukes aunt and uncle slaughtered, Alderaan destroyed in front of a Princess Leias eyes, Ben struck down by Vader, Han leaving with his reward, R2D2 shot as he sat in the X-Wing. These were very sad events, and we cared deeply about each of them. Not until the end when the victory was had and the well-deserved medals were handed out did we really celebrate.
Rey had the true feel of a Star Wars character: Alone, sad, abandoned, but also tough, practical, and extremely real. Her character went somewhere. She grew, discovering her power and responded to a calling that she really didnt want. So in the end it wasnt joy you saw in her face, it was duty.
To all those saying that her discovery of the force largely on her own, without training, was Mary-Sue-like: I think you are all somewhat missing the point. Part of what made Star Wars so captivating and powerful to so many people in the first place was this idea that there is a special power inside of people that is able to make someone ordinary and typically powerless (in the movie, Luke) into someone who is able to play a pivotal role in the world. The generation of kids moved so much by Star Wars saw themselves in Luke, a character whose dreams were bigger than his circumstances. With the force, though, he was able to lift himself from ordinary to heroic. And they were inspired by that.
J.J. Abrams may not have made a perfect movie, but he sure did nail that aspect in Rey. She's what made this movie work for me, and she's also why I'm so excited to see what comes next in the movies to come.