Infinity Ward actually does innovate.
CoD1: Takes the Quake 3 engine and the Medal of Honor formula and combines them in a new and refreshing way. Sure it wasn't the first WW2 shooter, but it brought so many things to the FPS table.
CoD2: Many gameplay changes (for the worse in my opinion) and graphical enhancements. Squeezes the last little bit of life out of the Quake 3 engine. Multiplayer is refined, though largely unchanged.
CoD4: Other than the setting, many gameplay elements are in tact. The multiplayer however, receives a complete overhaul.
MW2: Refinement of the single-player, and revision of the multiplayer. The lack of dedicated servers is a HUGE step backwards, but everything else is so polished I'll almost be willing to forgive them.
Treyarch doesn't innovate at all. Granted I did like Call of Duty: United Offensive back in the day.
