First impressions:
The graphics are damn amazing! Its definetly leaps beyond other games out right now (with the possible exception of gears of war), and its crazy to think that they keep the graphics toned DOWN for multiplayer. What they're holding back for the story mode is going to be insane. The warping effect from the bubble shield, the dust and debris that flies out from explosions, the physics, all top notch.
The gameplay seems a bit faster paced that halo 2, the firefights a big more frentic, but it may just be because everyone is getting used to the new gameplay style. A lot of the weaps have had adjustments, some things i noticed:
-AR of has 32 bullets in the clip instead of the "classic" 60 from H:CE, and a LOT more stopping power, but definetly only from mid-to-short range.
-Shotgun has only 8(?) shells in the clip, down from 10 in halo 2 (i think.)
-Brute shot has 6 shots in the clip, and can be fired in more rapid succession than in halo 2. It seems to do a bit less damage now, though.
-Plasma pistol seems to have a longer cooldown after firing a fully charged blast. It was hard to do the "noob" combo with a BR or AR because the weapon switch was delayed while it was cooling off.
I didn't get a chance to use the homing rocket launcher or minigun yet, but I did blast a few dodes with the spartan laser. Definetly a satisfying way to take someone out

Its pretty hard to use though, because unlike the plasma pistol, you can't hold the charge down indefinetly. Once it reaches full power it fires, so you need to be very accurate.
One thing that sucks so far in the beta is it seems to take a long time to get into a match. As a single player it would take over 5 minutes to find an appropriate game for me, but witha larger team the wait time decreased. There were also a few bugs with the game saying "the beta is over" and where it wouldn't download files from the server, but hey, ironing shit out like this is what betas are for, right?