So far, a few thoughts:
Weapons - overall, not much has changed, but the tweaks that have been made make each weapon seem to serve a clearer purpose than in Halo 3, where I felt some were just so rubbish compared to others (single wielded anyway) like the plasma rifle. Even the BR from 3 wasn't very good in mp as it too long to scope I found, the new version (or old, whichever you look at it) is immediatelly better, and in general the whole set looks far punchier and more fun to use.
Vehicles - same as Halo 3 as far as I can tell. The Warthogs still in, so I'm happy!
Level design - only a few on offer to try but they are alll good, a couple a little larger, but like 3, they are very different to the types you get in say MW2 or, especially BC2. There are less places to hide, and more fighting in the vertical axis. All the ones on offer are good though, I think SWord Base is my favourite.
Graphics / Presentation: presentation is, as always, slick and crisp. The new character designs look good, although I found the Elites don't look as good as I had thought they might. The in game graphics are better but not a generational leap forward. The frame rate is very smooth though and doesn't slow down in a big fight.
Sounds: same as Halo 3 really, although the weapons and explosives sound better. There's no music in mp so can't really judge well here.
Gamemodes: ok, this is where the game really shines. I am usuall a straight up death match kind of guy, but in this all the modes are fun and require genuine different strategies. Add the new "powers" into the mix and there are many more ways to play than before. The new Invasion mode is probably my favourite so far, requiring team work if you want to win...some people going in stealthy, some people using the hover packs and using a mixture of weapons.
The new in game credits system is also better than before, as you can customise your player a lot more. I would like to see more emblem variations though as it's pretty much the same as Halo 2 in this respect. I expect in the final one there will be many more armour types, I hope that there is some extra things you can buy to further augment your character, e.g. a genuine "custom" character for a specific game mode, where you could start with a different weapon and tradeoff speed for extra armour etc.
Overall I'm really impressed, lots of little tweaks to the already very impressive Halo 3 and ODST, and considering this is only a beta I'm already convinced this is going to be a great game.
