I think people are more excited at the prospect of just playing in that kind of environment... an MMO could have a lot of potential... you wouldn't just have zombies, you could have raiders, dogs would return to being feral, rats and other creatures.
The gameplay would mainly be scavenging, as there would not really be a lot of moments of shitloads of zombies, you could save those for major quests, but it would primarily revolve around establishing transportation, water and food supplies, ammo supplies, etc. Kind of like Dead Rising if there were less zombies and you weren't restricted to the mall.
The MMO aspect could be limited to major cities, so you join a group in a city like how WoW has different servers. You could also become a zombie yourself and then play it out that way.... if done right there's a lot of potential.