Yeah, CoD4 was kind of dark and ****ed up in all the right ways. MW2 was just... tacky.
The short "shock" missions are the best example, I think.
CoD4: Right after you save a downed chopper pilot and you're feeling awesome about your accomplishments, you get c**kslapped as a crazy terrorist detonates a nuke and you crawl around slowly dying of radiation poisoning.
MW2: Hey look, another nuke goes off...except now you're an astronaut...except you're not, because any rudimentary knowledge about how nukes work in space completely ruins it as you realize that the way things happen is complete crap. I can just imagine the dialogue:
Moronic Dev 1: Wait a minute...how do we top the mission in CoD4 after the nuke goes off?
Moronic Dev 2: Hmm...I KNOW! Put it in space.
Dev1: Space? But that has absolutely nothing to do with the rest of the game.
Dev2: Dude, who cares. Space is like bacon, it makes everything better!