I've noticed one BIG difference in people's behaviour between MW2 and MW3 when enemy killstreaks start popping up.
In MW2, everyone* runs and hides in the nearest building where they are picked off by the enemy team. This is particularly true of Harriers.
In MW3 the killstreak lasts all of 4 seconds before someone shoots it down.
In both cases no-one runs the perk that stops enemy killstreaks from killing them (Cold Blooded / Blind Eye).
So why then, in MW2, does no one make any attempt to shoot down enemy killstreaks to stop them dominating the map? It's not as if it's hard to do. I even have this class set up specifically to do the job (as well as move around stealthily and engage enemies at close-mid range).
Stinger <--- Shoots down killstreaks
Cold Blooded Pro <---stops killstreaks from killing you
It boggles the mind sometimes - when people destroy my killstreaks (it's rare) I get annoyed but accept it's part of the game.