I'm amped for Headhunter, although the other two new modes look pretty epic too. But yeah, Headhunter looks screamingly funny. Not too interested in the Arena, though, since I'm not much of a super serials gamer. I've always played in the Halo 3 social playlists. Actually, the overwhelming majority of my Halo time is in local split-screen with friends. We've kinda got a regular thing, jamming random weapon Slayer, Oddball, and Rockets. It's such a stupid game, really, but it's so much fun.
And although it's not in the beta, I'm also looking forward to seeing what they do with Firefight in Reach. I know they're doing something with it.