Your question caused me to pause and really think about my answer. I can't figure out if my brain is answering as a Call of Duty fan, or a Call of Duty developer. It probably comes from a position of both.
Every game in the Call of Duty series is based around certain core values to the property, but each also has its own flavor and personality. It’s tough to directly compare this game to just one particular title in the series as each looks to build off of the greatest from the ones before, and to learn from the opportunities each presented. I think the campaign mode is bigger and more impactful than any of its predecessors. Innovations to Spec Ops, including Survival Mode, are really exciting and beyond either MW1 or MW2. From a Multiplayer perspective, most hardcore followers of the franchise say that Call of Duty 4 is the purest shooter in the series. Minimal air support, superb level design, gun skill based, etc. Modern Warfare 2 was fantastic in its own way as well. Customizable killstreaks, more weapons, and faster game play were some of the key expansions that made MW2 a very dynamic multiplayer experience.
We play Call of Duty: Modern Warfare 3 multiplayer every night at Sledgehammer Games. The game feels like it has gotten back to some of the roots of why this series has been so successful. It’s a revamped engine with a refined approach on features. Our Community Manager, Guy Beahm, is a rabid high level MP player. He believes MW3 has taken MP to the next level, and I agree. (Or at least, I think so, since most of the time I feel like I am just running my head into his bullets.) There is elegance in the map design, with great flow and less verticality, a refocus towards gun skill, less noise in terms of air support, and teamwork is crucial for success. At least that is how I feel as a fan-boy/developer.
We understand that once the game is launched, it isn’t the end of it. The fans are the key for great MP development. Constructive feedback is a necessity, and openness to criticism is a requirement. Here at Sledgehammer Games, we’re big fans of Treyarch’s community model because of how actively they interact with their fans. They continuously engage with their community and look for ways to help define the best multiplayer experience for players.
We have the same intentions. We’ll be looking for feedback on how to give you the experience to date, and beyond, in the Call of Duty franchise. When 11-8-2011 comes around, please know we are all ears. Tell us what you like, what you don’t, and where you’d like to see things go in the future. While you are at it, please quickscope the %$#& out of Guy Beahm if you find him online.