CoD4 and WaW's complex map design and lack of any Launcher secondaries.
MW2's complex map design, powerful shotguns (as primaries), netcode and Team Tactical gametype.
Black Ops' good map design (not exactly complex), tomahawk (**** the other throwing weapons), non-stacking killstreaks, CoD Points system, Wager Mode, emblem and clan-tag gun customization, and equipment system.
MW3's Specialist, Infected gametype, Team Defender, and M.O.A.B.
Black Ops 2's weapon balance, pick 10 system, scorestreak system, and balanced perks.