I don't know if anyone has mentined this thus far, but the biggest problem with halo 2 (and Half-life 2, for that matter) is the devs promised SO MUCH during the E3 demos and delivered so little.
-What happened to AI kicking down/shooting through doors, using the environment and interacting in surprising and novel ways with one another? Half-life 2 was especially guilty of this kind of crap. Instead of getting fantastic and new AI, Halo 2 gave us Halo 1's AI and Half-Life 2 gave us retarded AI. Did it matter that half-Life 2 wasn't in black mesa anymore? Nope, cause you still walked down "corridors" of buildings in a straight line for hours on end, with maybe 2 open areas in the entire game. Halo 2 gave people plenty of free space (heh) but they didn't fix the "wasn't I JUST in this room?" syndrome that plagued the 1st game so badly.
Half-Life 1 is a great FPS (on PC, and sucked balls on the PS2) and Halo is the best FPS on consoles and at least a good game on PCs, but their respective sequels were boring and at times tedious.
Vista is going to become mandatory sooner than people think. MS has already begun cutting support for Win2K (AoE 3 is XP compatible only) and with all the pre-builts that people buy (OEM Vista, of course), there is no reason to believe Vista won't be adopted rather quickly.