If you're looking for a great alternative to the COD series, look no further than Battlefield: Bad Company.
Of course we have the usual elements of main storyline which meanders through a series of chapters until you face that bad guy at the end (I won't spoil it for you but it leaves the door open for a sequel coming out next month), with certain items such as gold bars and rare weapons, which does add a certain replayability factor to Bad Company. There is, of course, the intrinsic multiplayer too, which focuses more (at first) upon capturing the gold from the enemy instead of wading in to kill everything in sight, this mechanic works very well and results in a rewarding experience.
The chemistry and banter between the four lead characters is nothing short of hilarious at times, and comedy isn't the first thing that comes into your head when you think of a game based around war, death and shooting people in the face. While it doesn't make light of this, Bad Company doesn't take itself too seriously either. This in turn made it feel more real and in depth than any 'war simulator' that COD could offer me. I felt a sense of camaraderie and gritty realism that you don't get in a lot of games, never mind just FPS's. I felt involved, like one of the guys. Also, it's is the ONLY game which I have enjoyed so much that when those credits had finished rolling, I went straight back in and did it again on the hardest difficulty setting. It takes one hell of a game to do that.