The CoD stuff seems to be less about the "game" and more about the "wartime experience."
They do some pretty gutsy things with story telling in this one, stuff you do not expect, and there is
of the frantic feel of a really well done war movie, an incredible sense of scale (you are frequently pretty insignificant, just another soldier in a big conflict), really eschewing the whole "one guy saves the world" schtick of most FPS games.
There's a tone to the CoD games you just don't get in many (if any) other titles. My spolier above is just one example. You just don't see that kind of departure from the norm all that often. Even "perfect" games like Bioshock, you are the superhero, saving the day, and you'll live through it (probably) because you are the hero.
CoD doesn't let you make that assumption
And I think that makes it worth playing