From what I've experienced and seen, BF3 is 2/3 running around massive maps, 2/5 getting into a tank/helicopter/jet, 2/5 fighting, and 1/5 waiting to respawn. It does look fun, but the gameplay looks far more... subdued.
MW3 is high intensity, quick action, but at a nice pace. Most maps are very well designed allowing every type of player to excel. Quick respawns and killcams are great, and the game is designed with slightly worse graphics, but no matter what happens on screen, frame rate never drops below 60fps. That's why it's not as detailed as BF3. However, BF3 basically sucks if you're on foot.
Just my opinions though.