I don't really have a problem with Halo itself, I have a problem with the fact that there are 6 games in the series (1, 2, 3, ODST, Reach, 4 upcoming) and they all look and play identically. I think it's possible to feel like Halo while still trying to make the series evolve, but it just doesn't. Basically, I played the first Halo, and due to that I don't feel the need to play any of the other Halos since they're not a different experience. I did play most of them, of course, but none of them anywhere close to completion.
As I said, it's a problem that seems to permeate the whole industry right now. It seems every new franchise does that now. If the first game is successful, they just re-use the gameplay and graphic engines from the previous one and change the story. I loved Uncharted 2, but couldn't play the rest since they were identical... for example. Either that or they get streamlined to the point of just not being good anymore.