I've spent about 2 hours playing the online campaign and agree with Nordwolf, it's basically MW2 revised and the way MW2 should have been the first time around in my opinion. The game play is very smooth and you don't have to unload your whole gun to kill people (unlike MW2). The maps overall seem a bit smaller than MW2 but are reasonable. Compared to BF3, they're much smaller. The graphics I would say are about the same. They've definitely spent some extra time on the sound effects as there is a huge difference there - the guns sound like cannons. They may have overdone it a bit there, but it definitely makes you feel like you have the power!
So after a couple hours of gameplay, it has my vote. If you like MW2, you'll love MW3. If you were looking for something different, then you may be disappointed and would probably be better off checking Battlefield 3 out.