It doesn't seem so "modern" anymore, because it was released in 2008 or so. The only reason I had bought it was because that's what everyone was playing at LAN parties. Although it itself is a sequel, this could be pointed to as the beginning of the series' sequelitis.
The single player campaign deals with the US Marines dealing with some coup in Middleeastistan, and some Brits in Russia doing a lot of little stuff. The missions alternate between the two, making it difficult to follow. At some point, nukes launch and blow up, and you start going after the guys responsible. If you can't tell, I didn't follow it well, nor was I that interested. It was also pretty short. I played through quite a bit of it several years ago, but stopped for some reason.
The multiplayer is alright. Unfortunately, the people I ran with always put it on hardcore mode, which I hated. At the time, I had just built a super awesome PC with a gigantic 24 inch monitor. Without crosshairs or UI, you have to judge where the center of the monitor is (and thus, where the bullets go), while playing a FPS. This happened every time: I see the enemy; aim; shoot; realize what I thought was the center of the monitor wasn't (aimed one inch too far left); adjust; die. In all my other games, having better hardware than a laptop increased my effectiveness, but this destroyed it.
This is probably the only game that I have that was outright published by Activision. All the other games that I have or like and belong to them is because they have bought or merged with everyone else. I don't particularly like or ever had fun with Call of Duty, nor any other Activision game. The only time I've ever come close was Guitar Hero 2. It seems like they take a gimmicky form of gameplay, and run it into the ground. I am baffled as to how they got as big as they have. Maybe it's because I've been a PC gamer, and all I remember were demos of their shoddy movie tie-ins. Give me another THQ any day!