Infinity Ward removed Party Chat from Call of Duty: Modern Warfare 2 to prevent players from cheating on servers or rig matches, but the resulting complaints have convinced them to bring Party Chat back in limited form for Modern Warfare 3, Creative Strategist Robert Bowling explains:
A major focus of Modern Warfare 3 multiplayer is allowing you to have a lot more control over how you play, looking at those restrictions, and doing away with stuff like Party Chat blocking that's super hardcore. It's about giving options. We will have places where competitive guys go where you might have restrictive voice chat. And then you have an option to maybe play that same mode without the same restrictions if that's the type of player you are.
The only place where Party Chat should be blocked is Search & Destroy and won't be blocked for modes like Domination.
Likewise, he details, in a statement with Kotaku, how Modern Warfare 3 will take a step back from air strikes and killstreaks and return to the original Call of Duty 4:
I feel like it's something we nailed with Call of Duty 4. We moved away from it a little bit with MW2, relying heavily on air support, killstreaks, perks, and stuff like that. Modern Warfare 3, very much [is] building up from that Call of Duty 4 mentality of gun-on-gun, fast-paced infantry gameplay...
The hotspots are a key thing [in] Modern Warfare 3. We went into each map wanting you to be able to turn a corner and know very easily that these are three—no more than five—places I need to check.
For the hardcore guys who have been with us since the beginning, that speaks volumes: the map design, the return to focus on gun-on-gun, all that stuff. I like to always make sure the hardcore players know we're building it up for them.