I don't get the hate either. It's a very underrated single player campaign. Yes, it's way too short, but it also has some of the best action sequences put in a game and feels like an 80s action film come to life. I really, really dig it.
It's single player was superior to Halo 3, as well. If it was just a single player game, I'd dock it a couple of points, but coupled with outstanding and expansive multiplayer, it's one of the best games to come out in the past few years.