I see a lot of reviewers dismissing MW3 as just a small update, the Madden 12 of FPS so to speak. In my opinion, this is not the case. The fundamentals of the multiplayer have been changed and the story that was started in COD4: MW has finally been resolved.
While the maps seem similar, the weapons are mostly recycled and the graphics haven't gotten an update there are a lot of changes that make this a completely different feel from MW2. The kill streak rewards are the biggest change, and not only are they different, they've completely changed the way a match can play out.
There are now 3 different types of kill streaks: Assault, Support and Specialist. Assault is the standard; you kill, you build, resets when you die. The other two are completely new and have totally changed the feel of the multiplayer experience. Support counts kills over the course of the MATCH. Death no longer stops your kill streak growth bonuses, meaning that you can earn the top tier kill bonus on every match (if you accumulate enough kills). People are no longer punished for a cheap death or a stupid mistake, and can help change the course of the match for their teammates without camping off in the corner hoping people stumble by.
The third type, Specialist, is even more unique. It unlocks perks as you kill. This means that after a few kills you can have MORE perks active instead of only three, making you a far more formidable foe (so long as you know how to USE the perks). Once people figure out the sheer options they have for multiplayer, believe me, you'll be noticing a serious difference.