I don't see it that way. The film doesn't exactly end on a cliffhanger, but not everything is resolved. The First Order is in a far superior position to the Empire at the end of ESB, because the Resistance has been almost completely destroyed. There are, what, 100 people left in the Resistance? Fewer? They have, as far as we know, one ship: the Falcon. Literally. All the fighters? Destroyed in the hangar bay of the Raddus. All their capital ships? Gone. Destroyed by the First Order. All their bombers? Lost by Poe's stupid mission. All the materiel, all the weapons, everything has basically been left behind, except what could be loaded onto the Falcon. Maybe there are some hidden caches of equipment scattered around the galaxy, and maybe those outer rim allies will start to help, but the Resistance/New Rebellion is basically down to a handful of people.
On top of that, there's the much more interesting question of where the story goes from here in terms of the fate of the Jedi. What will Rey's Jedi order look like? What will it mean to be a Jedi the way Rey does it? Will the cycle that has been repeated in the previous two trilogies and into this third one be ended? Will Kylo Ren/Ben be redeemed? Will Rey destroy him? Will he destroy Rey? How will the FO be stopped? Will Poe be able to lead the Resistance now that he has learned from the errors of his past? Will Finn let go of his hate and fight for love instead? Does he have a future with Rose? And what of the children on the casino planet? What of Luke's ominous message of "See ya 'round, kid..."? Will Luke impart a final lesson to Rey? (There was a scene shot that was cut, which leaves that a possibility still.)
More importantly, I think the big question of "Where does Star Wars go from here?" needs to be answered. Will we abandon the notions of bloodlines and destiny, and take a new view of how the Force and these stories function? Will we continue to iterate the same kind of stories as the original trilogy, with yet more "evil empires" and stand-ins for the Sith/Jedi? How will all of that play out?
I don't think it matters, honestly. In fact, I think that's one of the underlying messages of the film: we need to be bold enough to move on from the past. Not to disregard it entirely, but also not to allow it to hold us back.
Listen to Yoda's message: "We are what they grow beyond." I think this is one of the most important messages in the film. Possibly in the entire franchise.
I think she absolutely did. Others have discussed it here to some extent, but I think Rey was searching for information, questioning her own identity, questioning all sorts of things and looking outside of herself for someone to give her a solution. What she takes away from Luke on Ach To is very simple: stop looking to everyone else to solve your problem, and fix it yourself. Even the "dark side cave" shows this to her. Rey asks who her parents are. What she's really asking is "Why am I this way? Who is responsible for me?" The cave shows an unending line of...herself. Rey is responsible for herself. Rey is the reason she is this way. Rey holds the key to her own destiny. Not some master, not her parents, not a wise sage, not her adversary nor his master, not some mysterious cave.
Both Yoda and Leia say a similar message, in that Rey and the Resistance have everything they need. Rey will chart her own course now, as opposed to looking to other people to tell her where the path is, and how to walk it.
So, the above mostly covers my own attitudes regarding the "Mary Sue" argument.
First, the classical "Mary Sue" is actually a form of author wish fulfillment, where the author places themselves in the "Mary Sue" role, and is basically perfect. The character has all the answers, is good at everything, never fails, saves the day, is beloved and respected by all she meets, and seems to be held in much higher regard or promoted beyond what would normally be expected. The character is usually female, too, which raises some questions about inherent sexism in the concept, although there's the male "Gary Stu" version as well so it's not as if it's only women. Wesley Crusher, for example, is seen by many as a "Gary Stu" for Gene Roddenberry, in that he is this boy genius who realistically has no business being promoted to acting ensign, who likewise has no business being taken seriously by the ship's command staff, but somehow is always given a fair hearing and very often manages to save the day. Obviously that changed over the course of the show, and Wesley became more nuanced, and failed at times, too. But he's an example of a "Gary Stu"/"Mary Sue."
Rey...is not.
I think people are hung up on the notion that Rey's power must come from somewhere. It does. The film even explains it. The Force manifested in her in a way that was much stronger than anyone else. Rey doesn't understand it, and has shaky control over it (although she improves her control throughout the film). She doesn't know why she's this powerful or why this thing awakened in her. Luke tries to explain to her that the Force basically "chose" her, but Rey still wants more info. She asks who her parents are, in the sense of "do they explain why I am this way?" when she goes to the pit/cave thing. The pit responds by showing her...herself. She is who she is because the Force chose her, and for no other reason. She's not a secret Kenobi, she's not a Skywalker, she's not anyone. She's a nobody, born from nobodies. But she's a nobody who was chosen by the Force, who heeded the call when it came time, who helps those who need help because it's her instinct. Arguably, that's why the Force chose her. She's a being of good, in spite of the upbringing she had.
Seriously, people need to stop and think about that. Rey has every reason to be filled with hatred and anger. Her parents abandoned her, indeed, sold her to Unkar Plutt for drinking money. Nobody (as far as we know) has helped or protected her throughout her life, except Rey herself. It's been her against the galaxy since she was six years old (or so)...and yet when BB-8 comes along looking for help, what does she do? Does she turn him away? Does she say "Life's hard, droid. You're on your own"? No. She moves to protect BB-8 and shelter him. She could have sold him to Unkar Plutt, too, but she doesn't. At many points in the story, she could've said "Screw this. I'm looking out for me." And each time, she doesn't. She chooses to help others. Arguably, that is why the Force chose her, but it's never overtly explained.
I think one of the main messages of the film is that the Force could be with anyone. It's not about your bloodline or destiny or prophecies. All of that stuff just leads to dead-end hero worship, where people turn to someone else to solve their problems, and instead, this movie is saying "Take heart! You can do it yourself! You have that in you!" That, to me, is ultimately the message in movie as a whole. The cave apparently represent's Rey's greatest fear: that she has only herself to rely on. But she conquers that fear and learns it isn't entirely true, either. She learns, in my opinion, that she can rely on herself -- that she's always been able to, really -- but that she also has a family in the Resistance. They're what she'll fight for.
So, anyway, no, Rey is not a "Mary Sue" in my opinion, merely because she doesn't come from a famous bloodline (as far as we know). I mean, Kylo Ren could've been lying to her or masking the truth, or just telling her what he thinks she needs to hear. But even if it turns out that she's a Kenobi or something, I think the ultimate message is that that's not why she's powerful. She's powerful because of her choices and who she is as a person unto herself, not her bloodline or her destiny. I love that message in the film, and I hope it's one that JJ doesn't step on in the next one. To me, it's such a powerful, important message, especially at this point in history.
