User blog comment:Argorrath/CoD Devs confirms Extinction DLC for future/@comment-4778612-20131216205851/@comment-4778612-20131216210027

It's just with the AI and the class system, the combat is a lot less repetitve, more action packed and requires more co-ordination between players to play, whereas in Zombies, the AI is basically "Oh, there's a gun! Allow me to put my face in front of it!".