So far my impressions are that the multiplayer is pretty solid, and a lot more balanced, but the netcode is shithouse. Far too often I die when I get the first shot in, and far too often I am shot when I'm clearly behind a wall. These have always been issues with COD games (and most FPS in general) but it's mega-bad here, and I'm talking on full green bars here.
The maps are pretty good, there's like four snow maps which is a bit overkill but I can't think of one I actively dislike. That said, the spawns suck ass from a straw. Many a time I've been killed by someone who has spawned behind me. It's actually pretty confusing, as you never really know where the enemy is coming from because the spawns swap so much. The map design itself is good, not much room here for snipers since most maps are middle-sized, but I like how each room has multiple entries, which helps discourage camping (which is still an issue though). One or two larger maps would have been nice though, but there's always map packs.
Personally I like the way MW2 'played' more - netcode was better and firefights were less guesswork. I also prefer IW's sound design much more - I like the music here but boy oh boy the guns and grenades sound pissweak. And the final killcams are not as good now, I think it's because in MW2 it still captured your movements after the kill whereas now you just stop dead, and it looks janky. Visually it's not quite as good as MW2 (that game had some really impressive textures) but I do like the colour contrasts as well as the improved animation. The actual package is superior here - much more balanced, discourages camping, and stats galore. Theatre mode is pretty comprehensive too which is nice to see given this is the first effort we've seen in a COD game. Single player I haven't touched yet, but I will say Dead Ops is surprisingly deep and just as fun as Nazi Zombies was in 2008.
I liked MW2's reward system - it felt more rewarding than just buying stuff (particularly since by level 20 or so you have a surplus of money - I just prestiged and reached nearly 100 grand) but at least the challenges here seem generally achievable, whereas in MW2 they were a joke. Contract system is nice, but again, not really worth it given how many COD points you get each game. Weapon selection and attachments seem pretty great, I haven't tried everything yet but nothing so far seems overpowered. Killstreaks too are well balanced, though the chopper gunner and gunship are rather ridiculous. Actually getting killstreaks is tough too, since they don't stack and getting multiple kills is harder due to the netcode.
Overall I'll see how things go, I haven't even touched Search yet since I've wanted to explore all the maps and get to know them in TDM. Search is what I was born for and I can see that these maps are well-designed for it. So I think my slightly meh opinion on the game may change once I get stuck into that. But for now I love the package but prefer how MW2 felt. Shitty netcode FTL.