Well I really like the upgradeable perks, and I think having different killstreaks is more interesting as well. And I'm very glad that martyrdom is a deathstreak perk now (martyrdom in hardcore was just mean). I also feel there's more 'balance' between the weapons, though still not on the level of WaW imo.
I haven't played single player much (maybe 5 missions in?), though I like Spec Ops a LOT. It's perfect for playing with friends who don't have the time to go through an entire campaign with me (though I wish there was also normal co-op)
My main gripes, matchmaking in the new one seems worse. I'm still sick of joining games that are in the last minute, and my team is down 20 kills. They've had enough time to fix that, and it's really upsetting that they didn't. And then the removal of the beginner matchmaking list. Instead, right from the get-go, I had to play with people who had already gotten to prestige mode (I got the game a little later) and knew the maps, and I got schooled. It wasn't fun at all, and I've been working to 'fix' my K/D ratio since (I'm obsessed with K/D ratio)
I've only played CoD MW1 and WaW, but these maps seem to be the worst so far. MW1's maps are far superior imo.