I have played Halo before and the reason I don't like it is because it's bland. It's not bad, but it's not great either. It does very little to immerse me into the experience. I just feel like an unwelcome guest in Master Chief's head. There nothing that happens in the game that stands out to me. While CoD: Modern Warfare, for example, knocks me on my ass when a nuclear bomb blew up in my face and I pulled my broken body out from the twisted wreckage of an airplane. When I look around I see nothing but absolute carnage, and the sun is blotted out from the sky. As I try to walk, I fall to my knees, and began to crawl before the ultimately radiation kills me. This kind of impact is what Halo lacks.
The single-player campaign is also too short. I'm one of those people who does not like to play multi-player games, so a game has to stand out, in my eyes, more on the single player content before I would even touch multi-player. It falls flat on it's face in single player mode. The story is unimpressive. The vehicles handle worse than the ones in Battlefield: Bad Company, and that's saying something. If you really think about it Cortana is the superior character. She can take over the weapons system of an entire city without anyone setting foot in it. She can remotely hack into any system she wants without trying, and could probably win the war just by herself. When it comes down to it, Master Chief is just a meat head grunt that is risking his life for no apparent reason. This begs the question: why does she need Master Chief at all?
I do have to admit though that I did have some entertainment when I kept running over my co-op partner in campaign mode. The first few times it was by accident and then I started doing it to piss him off. I admit it was awesome when I did manage to squeeze a GHOST into a building through a small door (challenging but not impossible) and kill everything inside. However, when you get down to it Halo is about space marines fighting aliens. It was been done before. It was not the first nor will it be the last. I have seen games that have done a better job at this genre such as Mass Effect.
tl;dr: watch Yahtzee's Zero Punctuation review of Halo 3: http://www.escapistmagazine.com/videos/view/zero-punctuation/8-Halo-3 This sums up my feelings.