1. If you think about it, in order to truly understand anything Tolkien says, you'd have to read the Similarion to understand anything he talks about.
2. The language he uses is very complex, confusing, and far more difficult to read than the Hobbit.
3. His story lacks one crucial element that is sort of my pet peeve: Character. Characters in lord of the rings are plain flat. Gandalf himself merely does exposition most of the time. His characters have no emotional depth as far as I could see. That's why I favor the movies. Peter Jackson managed to give character to Tolkien's characters, and made the story make sense to the best of his ablities.
So, that's really all I have to say. Keep reading my friends!