Okay, Jim, assuming you read these comments...
I don't play multiplayer anything, so I've no opinion on CoD multiplayer or CoD multiplayers, but I did play the SP campaign of CoD4 and found it severely lacking. It was so bad that I almost didn't bother completing it at all, but fortunately it finished just as I was about to quit it. Normally I'd complain about such a short campaign but in this one case I was relieved. It's the only FPS I own that I've not replayed.
I basically have two (related) problems with the CoD4 SP campaign: 1) respawning enemies and 2) what I like to call "scripting the player".
Respawning enemies suck. Shooters are about shooting things. The whole point of shooting something is that it dies. If the devs bring it back to life mere seconds later then the point of shooting it is almost entirely removed (indeed, much of the SP gameplay is about figuring out where to run rather than what to shoot). This is bad enough in any game, but in CoD it's especially annoying because it ties in with the scripted player problem. "Scripting the player" is what I call it when a game presents an apparent choice to the player (e.g. shoot the guys to the left and flank left, or shoot the guys to the right and flank right, or go down the middle) but only one of those choices is actually viable (e.g. because if you shoot the guys to the left or right and try to flank then they'll respawn before you're halfway to safety). These fake options are just insta-death traps dressed up in pretty graphics and animation.
Fighting through CoD4 I never got the sense that I had achieved anything. The only way to really "kill" an enemy was to pass whatever invisible line caused him to stop respawning - which didn't much feel like a victory to me - and finding the right "tactic" for any given section was just trial and error, not any great tactical planning or quick thinking on my part.
Do I look down on people who like the SP campaign? Yeah, I suppose I do. They strike me as easily fooled. I want games with genuine gameplay depth, but all these CoD players are voting with their wallets for shooters where kills have no meaning or purpose, and even the player's "decisions" are made in advance by the devs.