I can see why people are bashing linear fps.. But for an fps game non linear narration can quite hamper the gameplay/story.. look at these god awful games Crysis/FarCry 2... COD is much better than these games even it has the 2002 linear gameplay..

Crysis just places some checkpoints at random locations and puts enemies (with their super sharp guns that can shoot you from a god damn mile) all along the place/route and you die some 4 or 5 times before reaching your destination.. although Far Cry is pretty good... and for FC2 dont get me started. you take a mission from a particular faction and these faction somewhere camping on your route starts shooting at you..wheras the linearity in COD makes the storytelling more important.. This is just for comparison.. although i consider each game to be very good...