They added pointstreaks. Black Ops 2 just took that same concept and polished it. The idea with rewarding objective play originated with MW3.
They also added several new game modes, such as infected, drop zone, and All or Nothing, which were all huge successes.
Support and Specialist were brand new in Modern Warfare 3.
Proficiences were added as well.
They created the new demolition spawn formula too.
They added weapon levels to level up your gun and unlock everything. Now Black Ops 2 has that same exact system.
Faceoff was brand new with Modern Warfare 3.
The only step back I've seen/heard is that the scoreboard was different and didn't include objective play.
Modern Warfare 2 added the whole new killstreak system, instead of having the standard 3-5-7.
Modern Warfare 2 also added demolition for the first time.
Pro Perks were new in MW2 as well as the riot shield.
Bouns points for payback, buzzkill, afterlife etc.. all originated in MW2.
Black Ops 1 is just a polished version of MW2.
I'm sure there is more stuff, but I can't think of all of it right now.