No UI - No radar, no ammo counter, no health bar, nothing.
Few weapons, which deteriorate quickly.
Very little ammo.
Powerful zombies.
You can not carry much, and how much you carry changes how long you can run for.
Health that doesn't regenerate, and if you're wounded you constantly lose health from blood loss.
Not the kind of Western Zombie Game, where zombies are constantly going after you - The "Silent Hill" kind, where there's almost never any enemies, but you're always constantly paranoid. Although when there are enemies, there are many.
All in all, the game would have to be a total brain-asploding scare to be the perfect horror game. The protagonist should be the kind of person who dies in the first few minutes of a zombie movie - You're pretty much fucked, but you have to deal with it. Something that would be pant-wetting scary to play in the dark on your own.