There's really nothin' new to see here...
What to say here? Activision had a nice franchise, which is now dying. If they don't CHANGE something soon, and i don't mean add some new maps and unlocks, their next titles will start losing ground.
Singleplayer-
Singleplayer Campaign on Call of Duty always was interesting. It was nice. But, now, it's getting tiring. I can't say it's terrible, it indeed offers some cool moments, but they have could done way more. I'm disappointed from a CoD campaign for the first time.
Multiplayer-
So much to say here or nothing to say. It depends... I can say that it's the same since MW, or i can start number the reason i believe this.
The graphics for a game of 2011 are just ridiculous, they should start Working, with capital W. The sound design is a product of copy-paste (as well as everything on MW3). The gameplay is mainly balanced, but it gets boring after some time, and it lacks of realism. The maps continue to be small like bathtubs, but it's ok for CoD. I mean, if you buy this game, the last thing you have to worry about is maps...
I really see no reason why this game should deserve its money. Have you seen what a Mod can do? Have you seen Project Reality of Battlefield 2, First Strike on BF2142 or Blitzkrieg on Company of Heroes? Mods can change everything with some good work, and make a game completely new. Modern Warfare 3 could be a bad-made Mod for its predecessor, Modern Warfare 2. I don't know what MW offers that Battlefield does not. I see no reason of existence on Modern Warfare. It's time for it to get buried in History, together with Counter Strike and others that have nothing more to offer to both gamers and FPS genre.
Generally-
It has nothing more to offer.