Probably the fluidity of movement for me.

I also like how impacts are handled. When you shoot someone, you really feel like you are hitting something meaty - a lot of shooters don't have the sort of impact feedback that COD does. ( WAW and Black ops especially, Squelch sounds when limbs fly off - or am I just blood thirsty ?)

and of course the soundtrack!

I don't really like how unrealistically accurate the weapons are though. Especially the handguns