Flawed multiplayer, but a fun campaign
People are talking about COD4's experience-based multiplayer as a groundbreaking achievement in online shooters, and I find much to disagree about there. Basically, as you gain experience for kills and accomplishing missions, you open up more and more guns and mission types.
There is a flaw with this system, which is that everyone starts out with a poorly aimed pea shooter of a gun and must unlock the ones that can actually wipe you out fairly quickly. These guns are in the hands of the skilled players, who DO NOT NEED the most dangerous weaponry. I feel the experience would have been more challenging in reverse, providing easy kills at the start and working upwards by providing you with more precise weaponry.