Why do so many gamers dislike Halo?

[badgenome]

Never played 1 or 2, so I'm not sure what I missed story-wise, but 3 just didn't grab me with its abrupt opening. I also generally don't like games where you do nothing else but shoot. Gears, for example, has a cover system lends a lot of tension to the game; Halo felt much more monotonous.