You can't release the same game every year with different guns, maps, and story with almost no new gameplay mechanics and market it as a full priced game. It's getting really old, and there is no way I'm paying over 20 dollars for Cod 4: The second expansion pack (MW3).
Basically it went like this: Cod, Cod 2, Cod 3, Cod 4, Cod 4: WW2 mod, Cod 4: The expansion pack, Cod 4: The expansion pack: Cold War Mod
And Now: Cod 4: The expansion pack 2
To be honest, you need to spend more than a year on these games and then change the multiplayer