Originally Posted by
exochaft
The huge different between Rey and the others you listed, as it's very important for someone being a Mary Sue, is if the world seems to bend around them and validate them.
You mentioned Superman, which would be the most likely candidate to being a Mary Sue... but being super powerful alone isn't what makes a Mary Sue, and I'll circle around to this at the end, as Luke demonstrates the other half the issue.
When you refer to someone like Luke, his background is heavily focused upon based upon dialogue and interaction with his aunt/uncle and R2D2/C3PO. You find out very quickly who he is, what his personality is like, what skills, what his motivations are, and eventually why would he even want to leave his current lot in life. Considering he's established a necessary worker for a moisture farmer on a hostile planet, you easily infer that he has at least some basic level of self-defense and survival skills taught to him by his veteran uncle, as well as he's likely handy with some tech and droids further amplified by his interaction with the droids. You find out multiple times during this introduction that his one defining skill is that he's a pretty good pilot, his motivation is to train at the Academy to become an official pilot, and he's already wanting to leave but feels stuck on the moisture farm... and he's kind of a whiny little kid, too. There's a bit of foreshadowing about his father, but at the time the inference at least indicates that the traits he demonstrates are inherited or impressioned on him by his father who is not in the picture anymore. Later in the film, he gains a mentor that introduces him to the Force, and while he demonstrates he can use the Force his aptitude is fairly low. As his adventure continues throughout the first movie, he spends most of the time as a burden to those around him as Kenobi/Solo/Leia are much more in charge of the situation... and only at the end of the film does Luke finally have a shining moment where he finally contributes something to the cause, demonstrating his growth. As a slight yet important aside, Luke never directly confronts the big bad, aka Vader, and Vader not only sets a goal for him to rise to but also is inferred to be something that is currently unattainable for Luke.
Now... all that I stated about Luke and shown in the first movie? Almost none of it is conveyed about Rey. For the entire introduction, all we establish with certainty is that Rey is a loner, she salvages stuff, and she's waiting for her parents. Where is the source of her of her skills and what are her motivations? It's never established or inferred, so the audience has to assume things. Without making this post too long, Rey demonstrates skills at way too high of a proficiency from what were ever told, and she's just awesome from the get go. Her motivations are all muddled, because not only do we not establish her motivations but we never stick to any motivation either. Continually through the movie, people with knowledge and skills are suddenly dumber around her and her insight shines (such as her figuring out the Falcon's issues and resolutions more than the man who actually ran/operated the ship decades before her). Similar with her powers, which Kylo (despite his training under Luke and Snoke) just gets outmatched at every turn and loses every time to her, and Rey suffers no backlash or side effects from using power that she shouldn't have nor exceed her expected capacity... Snoke even berates Kylo in TLJ that he got beat by someone who had never picked up a lightsaber before.
Could all of these attributes been explained within the context of the film? Absolutely, but the movie leaves so many gaps in logic and explanations, all while making the world revolve around Rey. Some have mentioned various things the movie could've done that would fix almost all the issues. One of my favorite is that instead of Rey waiting for her parents, have Rey waiting for a lone Jedi, who found Rey as a child and took her under her wing, made her a Padawan, and for various reasons she had to leave Rey alone to go on some mission and hasn't returned yet. It'd explain to some satisfactory level where Rey learned to use the Force to such a satisfactory level, basic survival skills, as well as her combat skills with a staff and a lightsaber (which are two completely different skills if you've ever trained those weapons, the skills don't translate well). If this was the introduction of Rey instead of what we go, we suddenly know way more about her and have information that confirms her motivations, skills, etc.
This all leads to the overarching reason why ultimately Rey is a Mary Sue and Luke/Superman/etc. are not: the overall script writing of the new trilogy is piss-poor. That's it. If they wrote her with more of a standard hero's journey arc in this new trilogy (like Luke had throughout the OT) versus a flat "always better than everyone around her" with almost no room for growth, we'd be having a completely different discussion. Doesn't excuse all the events beyond character establishment and motivation that always seem to work out and have Rey win, but it still points to bad writing being the main cause of Rey being a Mary Sue. Honestly, what frustrates me the most about this new trilogy is that all of this could've been fixed if they tried... but RoS basically doubles down on what makes the writing bad and just makes it irredeemable in that respect. RoS even uses the most common tropes found in fanfics: the protagonist is related to some famous/important person in the story, and the protagonist makes a noble sacrifice at the end to save everyone.