WaW all the way baby.
the killstreaks in it were NEVER overpowered unless you truly suck. in MW2 if you come in a match where you are being spawnkilled by a chopper gunner/AC130 you don't have the time to grab your stinger/javelin.
hardly any bolt action SR's

I wished IW would've included all the guns from COD4. then MAYBE it would've been more fun. (R700 and the first sniper rifle ftw)
shitty maps. AND NO SHIPMENT LIKE MAP
and zombies > spec ops
the spec op missions have hardly any replay value to be honest. zombies never got old for me, still play it today but it's a shame that's there only 2000 games in the playlist nowadays.
those things are of course my opinion, I do not intend to bash MW2.
oh and they made it waay too noob-friendly. a friend of mine played it for the first time here and he basically owned with the FAMAS and the beginning killstreak.