The main reason that Reach bothered me was because of Canon. When they first released the game, it had glaringly obvious contradictions to the established storyline. It felt to me like Bungie, who had thus far been so careful to make their games match the book series, had given the project to people who had never even read the books (except for a glance here and there for dates). It was like playing a game of a movie that's based off of a book or reading a poorly researched fan-fic. The fact that Bungie did nothing to correct this except say "the game is canon, not the books" didn't help either. FoR is and always will be in my top five favorite books, and the fact that they took the story and trampled on it disappointed me. 343i has since tried to make Reach more like the book with datapads and what not, but there are still some things that haven't been explained.
Another thing that disappointed me was the campaign overall. I expected to be playing as a bad*** member of Blue Team or even Red Team (who, btw, made it off Reach alive. Just sayin) and ended up with a not as bad*** Spartan III. It lacked the humor of previous Halo titles, and what humor there was seemed forced. The AI you were forced to play as a team with was crummy and teamkill happy to say the least. And not to mention that the game forced you to watch allies die in unusual ways, like Kat getting killed by a headshot with a needle rifle when her shields should have been up.
Needless to say, there were plenty of reasons why I dislike the campaign. However, Reach has grown on me. Like a fungus.