I'm with Inu... on the multiplayer part. I wasn't paying attention to the single player story; I think it's just an excuse to go blow shit up.
Guns are easy as hell to use (Compare the M4 from CoD4 to the one in MW2; or use MW2's AK47 - isn't it crap? Surprise! It's the same damn gun that kicks ass in CoD4.), multiplayer maps are too complex for CoD's simple gameplay (sorry, but I don't want to spend half the match looking for something to shoot), killstreaks are horribly, horribly overpowered (spend the match kicking ass, then get something that makes ass kicking even easier.), snipers are useless (the maps, blame them, and also the ACR), akimbo is horribly cheap, OMALOL (grenade launcher, reload, grenade launcher, One Man Army switch back to the same class, MORE GRENADES!), the nuke (it's annoying to have a game end early. It's even more annoying to lose to someone who wasn't going after the objective, or win because someone on your team was nuking)...
Oh, and did I mention the stupid matchmaking that hardly ever works, the lack of dedicated servers, 18 player limit, no choice in maps, and the lack of admin-controlled servers? Hooray for aimbots! And the game cost more than other games on any given platform. And the DLC, too. Hell, it actually costs money on the PC... yeah.
The game got my "Biggest Disappointment of 2009" award. Even more so than Twilight, because I actually knew that was going to be bad.
CoD4 is better. In fact, I don't call MW2 CoD6 or CoD4 MW, because my mind still refuses to believe that they're related in any way.