when is the zombie thing played out? now? maybe next year?
I don't know. People have been wondering that since the mid 80's. Vampires are played out, that much is for sure.
I agree with what I've seen said before: I think it's got something to do with the concept of socially acceptable massacre. As violent games become more visible as a medium, we as a society start to pay more attention to what it is we're slaughtering on-screen. There needs to be an endless source of faceless, unredeemable enemies to play as our targets so that they don't weigh on our digital conscience and/or offend some existing part of society.
(For example, as I was playing through the Assassin's Creed series recently I noticed the occasional pang of remorse at chucking some archer off a roof before I was spotted - the guy's just doing his job, you know?)
For a long time, the mainstay villain was Nazis (or Dystopian-Future-Nazis) but everyone's getting a little tired of that. There are hordes of slavering space-bugs, or characterless goblins. And there's zombies.
Zombies scratch a particular gaming itch. We want to face something like us, but alien. That's where the fear comes in. We like the risk of infection - "there but for boomstick go I".
At least AZS isn't an FPS. I like the simulation style of the game, I think it's a fresher approach to the issue. That little green square of dots on the screen? That's your game of L4D on the streets, and look how small it is against the coming wave - what I like about AZS is its sense of scale.