Personally, I think the Call of Duty series went way too fast on releasing all those games. It seems like yesterday when the first Call of Duty appeared. I was an absolute fan of that game, and was blown away by the gameplay and realism. But then the next game was released, and the next, and the next.. And it all felt like they were trying to suck out all the money they could out of this game to me.
But seeing as it doesn't cost me any money, I chose to try out the last in the series. And I'm glad I did. It has all the awesomeness from the first Call of Duty, but it's situated in the present day, with present weapons and tactics.
What I love in the Call of Duty games is that you're never alone on a mission. With all those other games, they always send you on a mission on your own because you're better than Rambo. But in Call of Duty, it doesn't matter how good you are, you're always part of a squad that really helps you out and does its own parts of the mission.
Call of Duty rules!