I don't think I'm asking too much...  Once the game engine is complete and the entire skeleton is in place, adding more levels isn't nearly as big a deal as you think.  The problem with a game like COD4 is that much of the rather sizeable budget went into developing the multiplayer portion.  Regardless of the budget, if half is spent on the online portion, the offline half suffers -- and vice versa.  In the case of COD4, they most certainly should have fleshed out each level a little more, added a bit more dialog to really dig into the story, and bridged the levels together with more content.  It really should have been an 8+ hour game and most certainly could have been had the developers spent the time (and money) on it.  They didn't have too, though, because the market snapped it up in droves and thus the signal has been sent that uber-short campaigns are A-OK! 

^well then I expect COD5 to be 3 hours long!

but if this game was 8-10 hours instead of the 4.5 hours it took you, would you have bought the game instead of supporting the developer? They focus on the multiplayer because the person looking for multiplayer (and its the majority) will buy the game and support the developer. these people will also enjoy the game much longer, I currently have 100 hours. 4 days 4 hours online