I'd say the problem is that it's the same game over and over with minor changes each time. Series was pretty much "complete" with Halo 2, or at least it should have been. Ignoring the PC release, the big thing about Halo 2 was Halo online multiplayer. The big thing about Halo 3 was, uh, online co-op (which should have been in Halo 2). Then the campaigns are basically the same thing over and over. It's like they go out of their way to make a shitty flood based level to make sure it's consistent. Still, I enjoy the games.
Don't know if I'll buy Reach because I'm burnt out on Halo multiplayer. I'm still pumped for the second or third final Halo game from Bungie.