Anyone bitching about COD4's singleplayer should play it on veteran, then you'll realize how well crafted it is. The thing about IW and the COD4 engine is that IW knows how to direct the fucking hell out of a game. Yeah, it's scripted, but it's scripted very well. It looks good not because the engine is pushing graphical features that do the work for them at the cost of resources, it's because IW knows how to paint an environment. It plays good because IW knows how to use the simple AI and get them to interact with the player that goes beyond what you normally expect, turning it into something more like Contra. I remember being frustrated at the solo bunker section until I figured out what the enemies were doing, then it clicked.
if you did play the game on veteran and still didn't like it why the fuck are you investing that amount of effort in a game you don't like shouldn't you go use that time to pick up chicks ffs