Thing about innovation is that many games STILL haven't caught up with COD in many ways, ways that are important to the player.
AFAIK COD still has the deepest MP design of any FPS, with loadouts, prestiging, weapon leveling, and the list goes on. COD has brought ground-breaking singleplayer level design that few have taken a stab at (the 130 mission, Speaking Russian). They are risk takers, but are wisely saving it for when it counts to fight off percieved competition.
If anyone ever manages to catch up we might see more innovation. And where other games introduce new stuff worth stealing, like Gears 2's horde mode, COD manages to take and make even MORE tense and unpredictable.