Well, I think you have to look at a couple key issues:

1) Call of Duty 4 runs better with graphics on par with Crysis.

2) Shooting enemies is not very satisfying. Enemies are hard to kill even on the easiest setting. It seems difficult to place a shot accurately.

3) The blood and gore seems toned down. Limbs don't fly off. Bodies just lay statically. This is very, very disappointing. In fact, it might factor into me not even buying the game.