I didn't really notice much respawning in COD4, maybe a bit in one of the levels where you are moving through a city with dudes appearing in the windows of buildings etc but not too bad.
I do agree that it's annoying however, it just artificially makes you advance and hope for the best rather than methodically dropping all the bad guys.
There was one level where you move up a hill searching buildings for that bad guy, a cutscene plays and then you need to get to some extraction zone. You had like 3-4mins to get there. On that bit I just literally ran all the way to the building by the extraction point and hid in a corner until the chopper turned up. Seemed a lot easier than fighting past waves of bad guys.
Regarding getting stuck on scenery, I nearly did in one place. I started swearing at the screen, but finally managed to get out of it, very slowly.,, I think it's acceptable though because there's no way every nook and cranny can be tested. Typically it happens in games where you can jump over something into a little crevice, the engine thinks you are sliding down a slope (very very slowly) and thus prevents you from moving out.