I didn't get to play the Beta either, but I've watched a lot of videos. Obviously it's not the finished product and there were only a few maps available, but I am worried about a few things.
1. Why is the assault rifle the default starting weapon? Yes, it's an improvement over the SMG in H2, but it's still worthless in any situation other than short-range combat. Of course you'll have the option to change this in custom games, but matchmaking is going to suck again.
2. All the maps I've seen look enormous. The best maps in the first two Halos were the smaller arena-type maps, not these huge outdoor environments they seem to like so much now. Hopefully there will be a good selection of small maps.
3. The announcer sounds like crap.
4. The camouflage is worthless. Waaaaay too easy to see the person. It should be more like it was in the first Halo.
Hopefully they'll get some of these things fixed for the final version.