I love zombie games (which there are remarkably few of even though it seems to be a popular genre), and L4D kind of fits the bill of what I'm looking for, assuming they ever release it. Although it sounds like they're kind of screwing it up by trying to cram all kinds of extra crap into the gameplay now, and delaying it all the time. They should just trickle that shit out like they did with all the other HL mod/games they released.
Anyway, I'm not sure about a MMOZS game. I could see it potentially getting boring unless players had some real and challenging quests. I mean, after you fortify a position successfully it could get dull. It almost seems like this backdrop is better suited to an action game to me. I'm not saying it couldn't be made to work, just that I can see the "I'm safe, now its boring" part occuring without some kind of tweak or twist on the gameplay.