The biggest problems of the movie for me were:
1. The length. 2.5 hours is fine if they're 2.5 great hours. They weren't. Lots of fat.
2. The misuse of Finn and Rey. Keeping Finn and Rey separate for the entire movie was a huge mistake in my opinion. Their relationship was the backbone of TFA. To give them three minutes of screentime together at the end was criminal, especially in light of how little their respective sidequests worked. (Jedi Island was a snooze, sorry. And the Finn/Rose side-quest was clunky and meandering. And their weird forced romance certainly didn't salvage it.) I was fine with them being separate for some of it, but they were together for what? The Millennium Falcon afterparty and that was it?
3. Laura Dern's character. her story was fine, and the kamikaze ending looked great, but why create a character out of thin air to do that? If they would have just replaced her with Admiral Akbar, every scene in that storyline would felt 10x more relevant and 10x more interesting. Instead I was just watching her scenes wondering why her hair was purple and why Laura Dern was in a Star Wars movie. And horsefeathering think how awesome seeing Ackbar kamikaze that ship would have been. The fact I'll have to just imagine it for the rest of my life because instead they let Purple Dern do it makes me so angry.
4. They never figured out a satisfying use for Captain Phasma. At least she sort of did something in this movie, but overall, she felt like a cool baddie they never fully delivered on.
5. they still haven't really figured out an interesting way to integrate Poe imo. Him being a totally useless passenger on that escape pod while his Resistance peers were being blown to bits pretty much summed up Poe in a nutshell.
6. I probably would have rather seen Rey and Kylo fight Snoke instead of the red dudes. I understand the argument against it, and I didn't hate Kylo's shifty little move to kill Snoke, but the red dudes felt sort of like faceless drones. You never felt like Rey and Kylo were out of their depth vs them.