Here's how the Call Of Duty games are being separated;
Infinity Ward's Call Of Duty's are more about the Multiplayer. They tend to have the better multiplayer than the Treyarch games, while Treyarch's Call Of Duty's are more about the Zombies mode.
At first, it was a cool little minigame in World At War, but then in Black Ops, it started to grow, with the Easter Eggs becoming more and more elaborate, and the levels becoming more detailed, adding more and more to this very obscure storyline. We went from an abandoned building in the middle of Germany, to a lunatic asylum, to a swamp in Japan, back to Germany, now in an abandoned lab where the Zombies were manufactured, before being shot through time to a Nazi Theatre in the 60's, while two US presidents, a Cuban PM, and the US Minister for defense defended the Pentagon. Then after that, we ended up in a Soviet facility, where they were hoping to launch astronauts to the moon before the base was over run by Zombies. Then several horror actors (and one actress) ended up fighting Zombies, and George Romero, in Serbian. Before the original four musketeers were trapped in some backwards jungle. And soon they were on the Moon. And guess who was there? You're right, Zombies.
Zombies has now grown into a very important mode in the Call of Duty series, with fans of Black Ops requesting that IW add a similar mode to MW3, which I have no idea if they did or not because I haven't played it. With the new Zombies including larger maps
and the ability for 8 player co-op, it's become as important to Call of Duty as SP, maybe even MP.
I skipped out on MW3 because I do not enjoy the MP as much as others do. But I will get Black Ops 2, for the fact it has Zombies. People will call me stupid for only buying a game for a single mode, but 99% of the people who buy Call of Duty do so for the Multiplayer, and the Multiplayer only.
Who else will be buy BO2 for the Zombies?