Here is my 2 cents:
1) I liked the HUD changes. I never did like the UT2k3 HUD.
2) Weapons do seem fairly balanced but I agree with another poster here that the sniper seemed a touch week. The models of them seem a bit better now IMHO.
3) Assault looks to be done better. You know have an into "movie" that shows you what to do vrs actually reading the objectives. The Convoy map is ok for bots, but I would imagine its more for with Humans. The other changes to AS are pretty good.
5) Love the dualies AR, but I have a love for all dualies weapons so meh

6) Onslaught is a lot of fun. Good game type.
7) Vehicles. Ok for me this was bitter sweet. I have had the chance to use vehicles way back since Shadow Warrior back in 1997. So with 6 years I have seen a lot of good and bad ideas come and go. Currently BF1942/DC have the ones that are just an all out blast to play. I really did enjoy the ones in Halo as well. However for the flying one, it just sucks. Sorry but the Halo Banshee controls and handles sooo much better. I know there was big thing about how Epic "copied" some of the ideas halo. And there was the funny thread on Atari's forums that showed them side by side. But the rest seem ok. Not a bad mix.

In all with the short time I had to play "Teh Demo" it was a lot of fun. More so then UT2k3 was. Will it be enough to get more humans on-line? I don't know. The biggest issue I see is all of the other top caliber games out this year: Stalker, Doom3, HL2, SS2, Far Cry, Joint Operations, BF:Vietnam. All of these may end up pulling bodies away.
Also its too early to lay out any plans for Chaos and UT2k4. We need to play the real game as well. One thing that now can happen is our Hoover board idea that we have waited so long for....