If you own MW2, don't buy this game.
Anyways, MW3 is the EXACT SAME GAME as MW2 with different maps and a few small refinements that aren't even worth mentioning. My biggest beef with this game and franchise as it stands now is IW refusal to make a new gaming engine and give their fans a new experience to enjoy. IW are blatantly living off pass success and the COD name and turning it into another Guitar Hero.
MW3 changes nothing at all. The graphics and sound are the same since COD4 and the game really looks and feels dated. I can't imagine why IW refuses to change the game at all (hint: milking money for as long as they can).