"We believe a persistent world should be living, dynamic, and evolving. We believe your decisions should affect the state of the world and the state of the world should matter to you. We believe you should get to be the hero; not because you were able grind your way to some artificial achievement that everyone else will eventually grind for too, but instead because you achieved something unique and heroic -- something that people actually care about because it has an impact on the world."
In this post on Undead Labs' site, Phinney talks about how the company isn't trying to build a shell of an MMO around a mere action game, but is actually working hard to build a connected, persistent place where players have meaningful choices that could lead them to being heroes. His vision for this post-apocalyptic world is one that is "living, dynamic, and evolving" instead of remaining static.
And while the MMO will contain zombies, Phinney stresses that the core is about survival, not just a headcount. He sketches out some of the choices that will need to be made, including finding power, scrounging for food and developing safe zones. You can read more about this vision over at Undead Labs.