The Campaign is usually interesting, action parked and pretty! Key points stick in my mind from each Cod, such as the raids in Africa, assaulting Pointe-du-Hoc, placing the flag on the Reichstag, Sniping in Chernobyl, and jumping in during Market Garden. The original Cod having a special place in my heart for portaying alot of the scenes from band of brothers. It may be simple and simplistic at times but its fun!
The online scene is huge with a massive competetive community.
Replayability including customization and challenges.
Vareity of weapons, being both historical/realistic and they are balanced (mostly).
Its a simple forumla that works and its well....Great.
However I do hope they branch out into other conflicts such as Korea or Nam after MW2.