I still thing it's a really tough undertaking. There are no technical difficulties in this game, although there would be some semi-advanced tech like the physics for example. The difficulties would definitely come from emmersing and totally convincing the player.
I think a key feature would be total interactivity. I have this kinda vision for this game where you could be out in the wilderness (kinda like the original Night of the Living Dead) and you barracade yourself into a house. Like you said Van B, a physics system would allow you to slowly drag a couch over and block the door, but zombie AI would mean they'd relentlessly smash into it, moving it bit by bit (doors would also need health/damage so they'd also break eventually). You'd need to use the physics system to rig up a few items so the door was properly lodged closed. Perhaps a system that allows you to nail it shut, including windows etc.
I have this cool vision of the player with a revolver with 4 shots left and 50 zombies outside. Most action games you'd shoot 4 in the head then get ready for hand to hand. In this game, you'd save those bullets until they were right on top of you. I'd like to see the building filled with items and interactivity. Opening draws and pulling out kitchen knifes, hammers in a tool box etc. Maybe you can set fire to things to scare the zombies off and watch them burn.
Eventually you're gonna have explored the whole house and all you can do is watch the doors and windows slowly buckle and begin to break, so you'd better have planned your escape. Perhaps you've been upstairs and been ambushed by a zombie already inside when you were too concerned with house to secure the house - that would make you jump! And now you think you're only escape route is to wait until the zombie are right on top of you and most have filtered into the house, then climb out of the upstairs window onto the porch roof and jump (hoping not to hurt yourself and slow down your run speed). Then you're gonna have to run for that car you saw parked outside seeing as you found the keys in the kitchen draw. But it's gonna take some speed and dexterity to get that lock open, get inside and lock the doors and drive off before they're upon you again. If they catch up with you too quick, you're gonna have to leg it on foot in the darkness ... or maybe try and lead them away then get back to the car.
I mean, how awesome does that sound? How tense does that sound? And not a single shot fired, although maybe when you're about to get into the truck and only one zombie is close enough to get you, you'll be popping off a few rounds to aid your escape. I also really like the idea of the group member getting bitten. Do you cruelly execute him while he's still alive, or do you wait for him to turn? Maybe he was with you in that house while you were upstairs looking for wood to board up the windows and when you came back down, he came for you. Creapy stuff!
See, amazing possibilities, damn hard to code!
Edit: Oh, you'd also need energy, fatigue etc. It'd be tempting for players to just run around in the wildness dodging the slower zombies, but any pace higher than walking tires you out pretty quickly. You can't outrun them for ever. Also your player would need to sleep, so when that energy bar is almost empty, like the Sims, maybe make yaself a coffee to buy you some time, but eventually you're gonna have to find somewhere safe to sleep - somewhere where you can be sure you'll wake before the zombies are upon you if they get close. Use the physics system to balance a glass bottle on the door handle, for example. When the zombies come piling it, the bottle falls and breaking glass produces a specific volume of sound that will wake you, then make sure you have your weapon ready, or your escape route planned.
