It's so odd, to me Halo 2 and Halo 3 where not very good. I was dissapointed in those as Halo is probably my all time favourite video game.
I was never a big fan of the Brutes as enemies and dual wielding was also not very good. It destroyed the fine balance between plasma and bullet weapons, shielded and armoured enemies. I also didn't like the direction the story took, very convoluted and English speaking enemies...urgh...
I am a big fan of stories that exist only to get the game moving (portal, half life, unreal, halo) and give you a sense of purpose. The real story in FPS games are the environment changes, how encounters are handled, ebb and flow of action and non action parts..all in my opinion of course ; )
ODST took a big step back towards the original Halo, a good step. It re-established the "slow charging shield", health packs, single wielding weapons and it introduced a sense of mystery back to the series. In fact, I think ODST is the best Halo game since the first one. Well, until Reach came along of course because Reach REALLY is like the original Halo. Those fantastic Elite's are back and some of the vista's make me wish for a Reach engine enabled Halo remake.