I liked COD 1-3, MW1 because it was so different at the time, WaW, and Black Ops. To me they're like a Universal Studios ride where they give you a gun when you buy your ticket. Totally forgettable for the most part - I can go back and play them and not remember much of anything about previous playthroughs. I used to prefer Battlefield to COD in MP, because you could play your role and still have fun without being a pinpoint killing machine, but they have COD'ed that up to the point where it's not as fun to me as it used to be.
I like the WWII setting. Everyone got tired of it because they used the same set pieces over and over again. If you got your history from WWII FPS games you'd think WWII was only fought in France, Russia, and Germany. There were tons of other settings and battles which could have been explored, but devs and pubs stuck to Normandy forever with only a couple of memorable exceptions.
A WWI battlefield game would be pretty boss, fighting in fokkers and using those early tank prototypes. It wasn't all trench warfare.