It's another COD game, what did you expect? (It's not all bad, though)
Talking about campaign, MW3's campaign is just too short and made me question if getting the game mostly for the SP was a smart decision. There were lots of iconic scenes, but most of those were consigned to pure eye-candy. They missed opportunities to inject any creative gameplay. Story wise, without spoiling too much, I have to agree with GS in that IW dealt us the cheap card. Overall, it's a fun but all too familiar experience, but one that's more theatrically presented.
There's also the narrow FOV (nausea-inducing), FPS lock, and the usual terrible matchmaking on the PC.
Spec Ops and MP modes are really fun, but the matchmaking on the PC still sucks: random pings, you won't always find games on maps you like, hackers are everywhere, and dedicated servers are not ranked. All that might be less annoying/more tolerable on consoles though.
As a sidenote, people should stop worrying about the score the game gets (from GameSpot especially). Focus on the actual reviews, the points presented, and then decide if you'd like to get the game or not. If you're not sure yet, then rent it first.
As for the BF3 vs MW3, personally I think if you prefer actually playing as a team and riding around in vehicles, get BF3. If you prefer fast-paced, solo/deathmatch-like "team" play, then get MW3. If you think an older game does those two styles better, then play that game instead.