After much time thinking about it, I think I can trace the 6 things I hated the most about the movie, going in chronological order from beginning to end of movie:
1. Practically no action until the end of the movie. A lot of it is pure exposition.
2. The one bit of action at the beginning with the dune buggy was the biggest slap in the face to TNG fans. Picard would NEVER do what he did there.
3. The whole freakin' movie was physically dark. Can someone turn on the damn lights please!!
4. Shinzon reminded me exactly of Anakin Skywalker from the SW Prequels. A whiny bitch of a kid who is playing video games with actual war machines. Not for a second did I believe he was an actual threat to the crew. More like a whiny kid in need of a serious spanking. It didn't help that he looked nothing like Picard other than he was bald.
5. Data dies.
6. The whole wrap up to the story line is so horrible. Data's death feels like a huge defeat for the crew. The last 10 minutes are so depressing. Having B4 "hint" at being the new Data doesn't help. Then the movie has the audacity to play the triumphant music for the end credits, like we had just seen an amazing victory for the TNG crew.
I remember walking out of the theater thinking "WTF?" But I would have forgiven a lot of it if they hadn't killed off Data.