It's not the fan theories that I have the issue with, it's the stupid stuff. Super Leia in space, treatment of Luke, the entire casino scene, treatment of Captain Phasma, what happened to Snoke and not just what happened to him but this is supposed to be the baddest guy in the universe and he couldn't detect THAT!, Rey's parentage, Kylo Ren being a whiney bitch (although to be fair, this a Skywalker male trait), having more questions than answers from Force Awakens to Last Jedi. You know what, I'm gonna stop here before I end up hating the movie entirely.
About the stupid stuff you mention:
- Leia was Luke's first student in using the Force. She decided not to become a Jedi since she prefer the political side of things like her mother and adoptive parents.
- Like everyone, Luke is not perfect and has change over time.
- The casino scene was to introduce the kid who will probably be a Jedi in the future and to show that there's rich assholes in the galaxy making a profit over these "star wars" which we never seen before in the movie franchise.
- Captain Phasma is the Boba Fett of the sequel trilogy, so I wasn't expecting much from her.
- Snoke was a Sidious wannabe which we really didn't need. He was already physically weaken due to a prior incident and he greatly underestimated Kylo Ren.
- Rey not being a Skywalker or Kenobi is fine since it just shows that there's other strong Force-sensitive users in the galaxy.
- Kylo Ren is in conflict with himself like everyone else can be from time to time.
No one is perfect. Even Luke admitted that the Jedi weren't perfect. Across the galaxy, there are other strong Force users who don't come from a prestigious bloodline.