COD4 is simply awesome. I managed to sneak into the Beta, despite not being American, and fell in love with it there.
I have a few issues with the realism, as is usually the case with games. The grenades have much too generous blast radiuses. The fighter jets would never fly that close in, even when providing close fire support. The reload time for the light machine guns when you don't have "Sleight of Hand" is ridiculously slow (a person reloading that slow by the time they're being sent on deployment would be rightly beaten senseless by his or her Section mates). But it's a video game so I can't complain.
Anyone else obsessed with the music from the Epilogue mission? I can't stop playing that one over and over...