Ok I'll break it into things that are good and things that i think could be improved/added
Good
-Atmosphere is amazing
-Lighting is some of the best I have seen
-Intresting story
-Ambient music really adds to atmosphere
-Being able to shoot a door off its hinges
-The feel of the weapons (kind of hard to explain but I like the realism of using the sights with the aiming not being too hard.)
Improvements
-Change enemy character model (I read somewhere that you guys said it would be a bitch to do but do it anyway

)
-Possibly change the enemy's movement (I'm thinking more like the fast zombies in Half Life 2 instead of the overgrown Easter Bunny movement currently)
-Maybe an installer to make it easier to get going.
-I know it is probaly last on your list but custom menus would be sexy.
Additions
-Possibly in the survival mode have tables and lounges in some of the houses which you can push up against the doors and windows. This would keep the person safe from the monsters but they would still have to venture to other houses to get ammo where they would be vunerable.
-I was thinking that if the monsters had lived in caves forever, then their eyesight would be incredibly poor and would have been more used to hunting using sound or smell. This could be implemented into the game so that the player can crawl around and the monsters don't know he is there but if he shoots it could draw all of the monsters to the player's location. Also if a player is running away from the monsters he could perhaps throw a grenade somewhere to draw the monsters attention away from him. Also with teamwork, your partner could be somewhere else and shoot to draw the enemy away.
Note: I have never had any modding experience so I have no idea if these suggestions are practical or even possible.