My biggest problem was Luke.
Luke always seemed like the dude who always had hope, even when he shouldn't have. And for him to not have such hope here, just seems...out of character for him. The build-up and cliche reason he became bitter & mean just...didn't really earn this character arc...
...even if Kylo became like he did; it felt like it should've been more than that to make Luke bitter, IMHO.
Granted, Luke did what he could and played a bitter & miserable character well, it just didn't feel like that character arc was ever earned.