I dont know how much everyone has played MW2 but there is quite a few differences.
The Kill streaks, weapons, and tactical grenades alone have changed the game a good amount. Again thats if you really got into MW2 you should be able to see the difference.
The Range of knifing has been drastically changed from MW2 to MW3. Also there is different strike packages (assault, support, specialist) Thats a huge difference, since you really need to have a mix of different players on a team. As for the way the weapon fires, i think that has greatly changed (hint the proficiency) perks that are concentrated on firing, kick, and flinch.
I dono :iono:, i think if anyone had really came from MW2 to MW3 they would see a big difference in online game play.