The Enjoyable stuff: zombies, Zombies and more zombies. They really could make an entire game off of the zombies in this game. Add a couple things here and here. Infinite Replayability. Playing with friends is always fun.
The Same: Multiplayer. I never was a big fan of Blackops or Modern warfare after 2. It's different but similar at the same time. Lots of camping. Same song and dance. They added some customization between choosing being a perk head vs having a eccentric loadout of guns/addons.
The boring (IMO): The Campaign. I found it entirely forgettable and mundane.