I think the reason people didn't like Call of Duty World at War as much as Call of Duty 4, is because it almost completely copied and pasted from Call of Duty 4. It wasn't too bad of a game, but we've already seen that stuff so many times. Call of Duty 4 had brand new stuff.
World at War did have some nice new things like: Nazi Zombies, Coop Campaign, and some other things. But it wasn't like Call of Duty 4.