My only reservation is with the ability to veto. I've always, always, hated that in multiplayer games. Shy of a lobby where I can pick what I want to play and the map I want to play it on, vetoing is terrible.
Could we get a little detail in the OP? I didn't get to watch the conference, and Twitter has been super vague. It sounds like all the maps from Halo 1-4 are included and you can play Halo 1 or 2 multiplayer? What about 3 and 4? And can you play 1 or 2 on any/all maps?
Jeff Gerstmann made it sound like it might hit PC as well (on the Giant Bomb stream). When MS demoed it to him, he saw a "move mouse cursor to gain control" (or something like that), but when the demo person picked up the controller the text changed to "move left stick to gain control."