I loved MW1, I maxed out my rank twice and loved it both times, didn't like MW2 though, It was like buying a bike with no wheels.
however BC2 is, for me anyway, miles better than MW1.
The maps are larger, their are vehicles (including heli's) , it's more realistic than MW1 but still arcade feeling slightly, No more OP mortar strikes because you have to aim them and only the recon class can do them.
and as someone said here a while back, once you've witnessed building destruction MW2 will feel crap in comparison, It honestly adds another layer to the gameplay.