Halo 2 focused on multiplay so they could get that monthly xbox live fee out of you. But in the end the multiplay plays like a bad Unreal Tournament 2004 clone and they just screwed the best thing they had going for them, the intimacy. The cool part of Halo was how simple and small the maps where so you could smack around your buddy in your livingroom. When you make em' big and complicated for online play... there are already tens of online computer games that are much better, and you don't have to pay per month to play em.
Single player was okay but again, the ending was to force you to buy Halo3 so you have to buy xbox2 along with it. Microsoft thinks its so smart it can dupe everyone along in this strategy but when companies get too cute with consumers, the consumers usually see through it.