Some first impressions (multiplayer only; haven't gotten into the campaign or zombies yet):
-The flame-thrower attachment is fookin' sweet!
-Maps seem pretty balanced. There are some big ones, but they all seem to be pretty well thought out. Haven't really encountered a lot of camping thus far.
-Graphics are actually not quite up to MW2 par. They don't look bad by any means, but not quite as crisp or detailed. A lot of the maps are really colorful too, which doesn't bother me, but I've heard other people bitching about how it looks too "cartoony" for them.
-The CoD points and the wager system are kind of cool, but at the same time it's a little annoying to have use points to "purchase" guns and attachments and stuff despite the fact that you unlocked them via XP.
-You can pick up your Claymore and place it somewhere else, which is awesome. Also, you can have 2 grenades (a lethal and a flash/stun type) PLUS your equipment, so you don't have to choose between a grenade and a claymore like MW2.
-Perks seem pretty balanced. I like how they just totally eliminated Stopping Power and Juggernaut, so no more bitching about them. I like the way they rearranged some of the perks from MW2 into different tiers also. For example, in MW2 you'd have to choose between Marathon or Bling. Now Bling is "Warlord" and it's in a different tier, so you can do Marathon, Lightweight, and run an SMG with extended mags and a silencer. Pretty awesome rushing class.
-A lot of the maps have something going on in the background while you play (Launch has the rockets taking off for example), which adds to the franticness of it all. Pretty cool in my opinion.
Overall I'm pretty happy with it so far. Jury's still out on whether I like it better then MW2 though...