CoD started going downhill from the third game onwards imo. I think I remember hearing that Treyarch were going to do something different with the next game, hopefully it's going to be something that's going to make the game seem fresh again. It's just the same old same old lately. Treyarch did the franchise a lot of favours when the introduced Nazi Zombie goodness, so hopefully they're going to do something else just as awesome.
I don't get how CoD went downhill after 3, when it changed the direction of First Person Shooters forever. Modern Warfare became the blueprint for practically every FPS available in this generation, even now.
I can't stand the CoD single player, and I'd never buy if it was still shipped without Multiplayer, because the MP mode IS CoD these days...I'll literally spend about 2/3 days on the CoD single player experience, just to harvest the trophies and move on quickly.
Zombies was only fun when it was simple, imo. They went into overkill with the new gameplay features in Black Ops....Should have kept it simplified and not introduced all this bullshit with teleporters, pack a punched weapons, dozens of perks and massive maps with secrets and shortcuts.
Infinity Ward Spec Ops missions are quite good, but the new survival mode they introduced this year is just boring. It's OK for the first couple of times, but it's just your standard horde mode that you've seen dozens of times in other games.
CoD will always be about the Multiplayer as far as I'm concerned, and I'd actually thumbs up any planned notion to cut the single player completely if it means further focus on Competitve/Co-operative Multiplayer.