I played a few hours of multiplayer on my neighbor's Xbox last night, and thought it was pretty rockin'. We were more interested in trying out the new weapons and exploring the new levels a bit than actually
playing though.
Oddly enough, some of my favorite levels were the remakes; Battle Creek and Blood Gulch (or Beaver Creek and Coagulation, or whatever their new names are.) Multiplayer level design rules seem to be different; there are much fewer open spaces for easy vehicles and sniping. I won't miss the rampant sniping so much, but I loved playing good vehicle games. I used to really kick butt in a Scorpion on Blood Gulch.
I really like some of the new weapons; the brute shotgun, the Covenant sniper, the Covenant carbine; the Wraith's are fun too, although the "lob" nature of it's projectile is difficult to get used to. The ghost is vastly improved, and the Gauss gun variant of the Warthog really kicks butt.
I had seen preview videos of the duel-wielding and it really made me roll my eyes, but it turns out it's cooler than I thought. Not only is it
not a foregone conclusion that you'll want to do it (the tradeoffs of not being able to use grenades or melee attacks is pretty steep) but it's actually kinda fun anyway. Duel-wielding two needlers is pretty wicked from short to medium range. The sword was fun for a while, but didn't last. It's too difficult to use if your opponent has guns, unless you can totally sneak up on him. I can see it being really fun for certain multiplayer games, though. We used to play one we called "football" which was basically "plant your flag" with no shields and only shotguns (Longest Hall was the iconic level for that game) and I can see a variant of that with only swords being a really fun game.
Graphics were definately improved; gameplay was more of the same, but with a few more options and a better balance; you don't have as many crap weapons vs. really good weapons as you used to. Playing multiplayer with two random weapons was kinda fun too; it rocks to start the game with the rocket launcher and the sword.

The no health dynamic simplifies gameplay somewhat too.