Compared to...
Rey is still Rey from the end of TFA. Nothing more, nothing less. Nothing learned, nothing gained, and nothing lost (oh her parent were drunks.. Nah no credit given for that fail)
Finn is the same Finn from the end of TFA
Poe is the same Poe from the end of TFA no matter how much they tried and make him something he wasn't with the whole hot head plot.. Never got that feeling that he was a hot head in TFA... In fact he was
Luke is made to be bitter so he can be not bitter and give the rebels some time to escape.. Same amount of Time Finn would have given them were it not for Rose taking away the only true substantial thing (other then Snoke) this film had to offer.
Going back to this...Rey absolutely changed in TLJ. In both TFA and TLJ she basically sidelined herself from all outside conflicts (Jakku in TFA, Ach-To in TLJ) in order to wait until someone told her what her "place in the world/galaxy" was. She did find a place where she fit in with Han but he was taken away from her so she moved on to Luke. But Ach-To told her that life isn't about sitting around waiting for your "heroes" (whether they be parents or "legends" like Luke) to tell her where she belongs and what to do. She decided that she was going to take a proactive role in pursuing the convictions that were inside her. So she journeyed to Snoke's ship willingly (instead of being captured in TFA) she came to the Resistance' defense willingly (instead of via rescue in TFA.)
Everything kind of "went to pot" for the Resistance when Rey wasn't around and her being literally front and center in the last shot of the Falcon said that she's there to stay, and that going forward the Resistance will have the one thing it had been lacking--someone powerful in the Force (and the willingness to use it.) A huge change from the beginning of the film or from where the Resistance stood in TFA for that matter.
Finn and Poe aren't really main characters in this trilogy. They're more on the level of Chewie and the Droids or Marcus and Sallah in the Indy series. Their arcs are really only as important as whatever is needed to drive the plot forward, and to that they both fulfilled their respective purposes.
The "main three" characters in TLJ are clearly Rey, Kylo, and Luke. Luke's arc doesn't even need to be repeated, it's been discussed at length and is one of the highlights of the entire Saga. Kylo finally stopped being a poseur lackey (a primary theme of this film was growing beyond your teachers which all three did) and he became his own figure apart from Vader and even apart from Snoke.
So I wholeheartedly disagree with the claim that "nothing happens" in this film or that the *primary* characters didn't change or grow (technically even Poe did grow though he didn't really need to given his role in all of this.)
And I know that it's shocking for me not to concede that Rey is the worst kind of "Mary Sue" but honestly I've never in my life even heard that term outside of the internet. It's not like Rey commits all these sins of these past lame characters, starting with someone actually named "Mary Sue." Where does that term even come from? For most of the story she's lost and confused and largely directionless and in any other narrative those all *would* be considered "flaws." But for her they aren't because she can do a mind trick with no training, bla bla bla. I just think that all of those claims are largely exaggerated. Yes she's special, more powerful than others, etc., but again so is MCU Cap, Neo from The Matrix, Harry Potter, PT Anakin and so on and so on. Except she's actually more likeable than most other "chosen one" type characters save for arguably MCU Cap. Too many people are simply over-reacting and accepting the criticisms without really thinking them through.