Well, I'm not playing yet (waiting for the Xbone), but I'm mostly intrigued by stuff like this from the Reddit AMA:
We want to have a game where NPCs inhabit a living, breathing world, but where players make a real impact on the environment and become a part of that world. Through the thrall system we hope to introduce complex behaviours that allow you to build your own cities and towns and as we add features such as the Purge, we are going to be trying to create a dynamic world that challenges your right to survive
So, working with friends to build a town/fortress/whatever, that gets invaded by AI enemies, etc. sounds pretty cool. I know a lot of that isn't in there now, but it sobs like they're being given the resources to make it happen.