This isn't going to be very coherent. Anyway, the cast WAS great, I agree. They 100% found the three actors they needed to hold together these next 3 films. It is a very common criticism that the film drew too closely to Episode IV, which I will echo here. I do get that they'd rather play is safe with the plot and that doing things this way makes easing into the world easier for old and new fans, but it was unnecessary all told, I think. That said, it did not take away from my enjoyment of the film too much.
The villains in this film were underused completely, across the board. Phasma especially, but I would go so far as to include Snoke and Kylo Ren AND the general guy ("general guy" being a clue as to how fucking underutilized he is lol) This is my one gripe that I think it is excusable to say "they'll fix it in the next films", though. Also, Snoke as a big bad? Not buying it. He's a terrible Sith Lord. Palpatine was hands on, controlling EVERY aspect of his Empire, molding his apprentice to be the perfect weapon. Snoke is basically some guy. "Oh come to the Dark Side. Also do whatever, it's cool." Kylo is unfocused as a result, and his lack of training and discipline make HIS character less imposing as a villain, and serve against him in some key story moments.
Now let's talk about Rey. Get your pitchforks ready internet hate mob. How dare anyone criticize a female character in films, because we rarely get any of them and they are role models for young girls and anyone anywhere who has anything bad to say about them at all, these perfect snowflakes, is a sexist pig and we should fuck them up. Right? Wrong. Rey is a good character, but not without her flaws. I don't want to sound arrogant, but I think I've nailed down the whole crux of the "Mary Sue", Rey-is-too-good-at-things discussion. No, I do not believe she is "too good" a pilot or a mechanic. Why do I not believe that? Because the film presents a set of circumstances that explains her aptitude in those fields. She lives on Jakku, a desert planet, in a junkyard where she gets food rations depending on the value of the scrap she collects for her "boss". LITTERALLY IN ORDER TO SURVIVE she would have to know: The value (and therefore the function) of the scrap she collects AND how to defend herself against people beating her up to take her scrap and sell it as their own. The film EXPERTLY tells us without telling us that this is the case, her life depends on being a good mechanic/pilot AND being a competent fighter.
What the film DOES NOT TELL US is that she is knowledgeable in or competent with the Force in any way. We get hints, but that isn't enough. If the film doesn't present circumstances that show us WHY she is so good at the Force, it is not believable. Small things like tapping into the force to pilot the Falcon work, because the force is largely an intuitive power, and heightened intuition is almost a reflex for Force Sensitives. But it would take practice and understanding for her to do much else. TLDR? No, Rey is not "A Mary Sue". Yes, the film does a poor job of framing her as a viably adept force user.
How the FUCK did the REMNANTS of the Empire build a better super weapon than the Empire had WITHOUT ANYONE IN THE REPUBLIC KNOWING ABOUT IT BEFOREHAND