Halo 3 rocked IMO.
The one thing I noticed in the game that I REALLY REALLY loved was that I hardly saw any duel wielding people. Starting people out with the assault rifle is going to make match making MUCH more fun than it was before. It's a weapon that you can usually hold your own with in most situations when you spawn. And you won't see someone with 2 AR's against your 1 (like you did with the SMG in h2) so you won't be beat based on that fact.
I also REALLY liked the needler.
I wasn't really feeling the items. I think they are very gimmicky as a whole and don't add much to the gameplay.
I also think that Valhala will turn intot he new coagulation.
The shield doors on Snowbound are terrible IMO. They need to make it so you can temporarily turn them off, or make it so you can drop into the inside from atop the building. The time I realized just how horrible those shield doors are is when I played team ball on that map.
I also think the pistol sucks. It fires WAY too slow. I do like how the sword has ammo now. That is a welcome addition on my part.
Halo 3 is a GREAT step up from halo 2 as they seemed to have fixed a lot of issues I had (although the weapon respawn system still is terrible), but I agree with mzkhadir that it's not quite up to par with Halo1 gameplay. But that's understandable because the game is for the masses, not for the hardcore competitors only.
Last but not least, the sound in Halo 3 is AMAZING. I LOVE the sound in the game!