Rey and Ren in TFA vs TLJ
I despised Rey in TFA but really liked her in TLJ. We've all heard the Mary Sue accusations for Rey in TFA, and it's true. She could do anything, was successful at everything, had no weaknesses, no flaws, and always made good decisions. Mary Sue characters are BORING. But in TLJ she was interesting, she had flaws and made poor decisions. I like how she lost her temper against Luke. Her decision to confront Ren was very flawed but I understood why she would have done that. She failed to coax Luke out of hiding, but, with the nudge of Yoda, did breach his defenses. She was helpless against Snoke and had to be saved by Ren.
In short, I thought she was an interesting and relateable character.
I hated Kylo Ren in TFA. I thought he was a whiny emo sissy boy who was acting out. In TLJ, however, I really liked him. I could truly sense his inner conflict which made him a much more interesting and complex character. The interactions between he and Rey were the underpinnings of this movie and the foundation that made me like it so much.
Yeah, the rest of the plot was "meh" but the character development triangle between Luke, Rey and Ren was really interesting, imo.