in my opinion the thing killing the CoD series is the yearly releases....
wrong actually, theres 2 years between each game...:p
Keep in mind IW and Treyarch are two separate companies, giving them 2 years to come up with new ideas.
IW---COD4---2007 (I think...)
3arc---WaW---2008
IW---MW2---2009
3arc---BO---2010
IW---MW3---2011
(just thought id point that out:420