Simply put, Call of Duty 4 is one of the most complete and entertaining war games ever.
I began with the first mission and immediately I saw how amazing the visuals were. Light gleamed exactly like it's real life counterpart, and the human faces were near perfect. The more and more I played, the more immersed I got. Some people say the campaign is too short, but I tend to disagree. It seems to be a sufficient length for the casual gamer, and it certainly provides an in depth story line.
Gameplay, obviously, is similar to other games in the CoD series, but because it's set in a modern world, the guns have more power and it feels like you have more options. The levels are all fun and intriguing, and I've felt my heart rate jump into the 100's several times during some free for all shootouts.
All in all, if you are an Xbox 360 owner, you must buy Call of Duty 4. It's that simple. :)