While the campaign may have fewer levels than other FPS it is a spectacular story that is paced very well.
First, let me tell you about the campaign mode. The first thing I read in the Gamespot review is that it's short. I was expecting something that felt unfinished or rushed, but it was quite enjoyable and ended exactly when it should have. Recently I played through the Halo 3 campaign and it just seemed long winded. CoD4 gets right to brass tacks and has you moving all over the place in all the right ways. I never once felt like the campaign was too short.
The story itself is pretty amazing. Without going into specifics I can say its something that lets you feel immersed in what's happening. It's one of the most successful stories I've seen in both movies and games. Infinity Ward writes a story that is more true to life than maybe even what we see on CNN. I wasn't disappointed.
The graphics and audio are both top notch. Bullet casings fly all over the place and you can hear them hit the ground. (However, if you're listening to bullet casing you may want to redirect your attention to where your bullets are going.)
Call of Duty 4's multiplayer is the prologue part of the campaign. This is where the game has its value. The ranking system seems to work well, although prestige mode is more than weird. After attaining all 55 ranks the game will let you enter prestige mode. You receive a prestige 1 icon and the game will take away all your ranks allowing you to reacquire all 55 levels. There are 10 total prestige ranks, but with only an icon as motivation it seems more like an unnecessary time sink. Other than the small prestige annoyance the multiplayer is solid. It includes strong spectator support. Hopefully we'll see some tournaments on TTV.
If you know what F.P.S. stands for, you'll at least want to rent this one.