Played a few games. It was fun, but nothing new. Basically just Halo 3 with updated graphics and animations.
Not that there's anything wrong with that, if you're into that sort of thing. I'm sure I'll have fun with the beta, but I'm still not sold on a prettier version of a game I already own.
While it's very similar to Halo 3, there are a number of differences other than just looks. There are new weapons (e.g. grenade launcher, plasma launcher) and game modes (e.g. Invasion, Generator Defense, Headhunter, Stockpile).
Also, gameplay mechanics:
No equipment to pick up. Instead, there's the new armor abilities, which you can change at any respawn.
You have health in addition to shields, and health packs are strewn across the maps.
Only two grenade types.
Refined radar system.
See this video
to get an explanation of the various new features.