So I've played through this game now and its not very good let me elucidate on that opinion:
Story
Starting with the least important part of almost all games story, games can give you a story in two main ways. The first as a consequence of the gameplay, happening dynamically from the way you interact with the environment you exist in. Like in Halo, Left 4 Dead, EDF or in a more structured way in Mass Effect, Fallout, Deus Ex etc.
The other crappier way is to just paste a standard narrative based story into a game usally presented in cutscenes, text or pseudo-cutscenes.
Call of Duty, following a pattern of choices for most of the game, goes for the crappier one. On top of how the uninspired the method of story telling is the actual story is really naff like a substandard episode of Ultimate Force and very confusingly told; about 10% the game is a flashback to ten years before the events of the rest of the game (the Chernobyl levels).
But forget the story whats at the core of what make this game so naff?