My impressions so far...
Is it just me or are a lot of the guns pretty imbalanced? I agree with Panda about getting the DMR and Pistol balance wrong, and I think the Assault Rifle feels weak too. Either that or it seems to just take a lot longer to kill now. Unless of course you're using an explosive...at which point insta-kills happen too often. The rockets seem like I can get kills where I wouldn't at certain distances in Halo 3 and forget about grenades...why in god's grace are they so strong? And insta-kills with the grenade launcher if you land a shot directly on them? Meh, I guess...but it doesn't seem that hard to land the shots if you're hovering above them with jetpacks. And that thing comes with 7-8 shots! Any time I've done a Covie Slayer in Grab Bag it has degenerated into a sticky competition. That's exactly why I hated the Midship remake in the newest mappack...spawning with stickies is just not a good idea! And close range weapons seem to make quick work of people...if you get the hammer or shotgun on that one outdoor level (not the Sword Base one or whatever the indoors one is called) and guard the flag, they're just not gonna get it. That laser beam weapon (the sentienal gun on crack) seems fine with how quick it kills, but it's just that most 'normal' guns feel too nerfed/weak in comparison so it ultimately feels too strong. In previous games even if you ended up too close to a shotty/hammer/sword guy, you had a fairly large chance to escape or get a kill...I don't forsee many people backpedaling away with that ghetto Assault Rifle and winning in the beta, however.
I'm liking the loadouts though. All the abilities seem pretty cool and have their moments where they'd each be perfect, although I've been using the jetpack mostly just because it's fun to fly around like a moron. That guard powerup is pretty solid too...I've grabbed the sword on the indoors map, gone into guard and jumped out to kill 2-3 guys huddling around me a couple of times now and that's always fun.
Also, are headhunter modes and that sort only in FFA? I've only done Grab Bag so far and haven't run into that mode yet.