First of all, the game should be challenging. If you scroll through the entire game in a few hours without getting killed at all, something is wrong. I'm in favor of not putting an "easy" difficulty in horror games. People starting on "easy" usually get off too easily, which ruins the sensation of a first playthrough.
If you screw up, penaltise us gameplay wise and graphic wise. Only then, you'll be really motivated not to die, like you should be afraid to die in such a situation. Only allow us to save our games in specified areas or add checkpoints. If we die, show us how and gore and details are welcome. Might be unpleasant to watch, but that is the point exactly. I understand the people that don't want gore to be the center of the game; It usually goes hand in hand with games with big guns and stuff and its a bit of a cheap scarefactor. Also, gore loses its appeal if you're confronted with it too many times. Yet, dying can be displayed as gruesome as possible if you ask me.
Each enemy should be a serious threat. The moment you're a significant bigger threat than the enemy, something went wrong again. I personally don't understand the appeal of running away an entire game, but i wouldn't mind certain parts of a game where running is the best;only option. It also varies the gameplay somewhat.
If you're going to implant mobs; make them unbeatable. the moment I can mow down an entire mob is the moment I start to scratch my head on how likely that just was. After you've shot down an entire mob, the next one won't feel like a challenge. (Resident Evil 4) I personally think that "mobs" are perfect enemies that should be avoided.
Good voice actors are a must! don't think i should explain this one.
Have no safe areas. Resident evil 3 did this really well in my opinion. In most games, you feel safe because you either cleared the area already, or because you walked through it several times and nothing happened. Resident Evil 3 had Nemesis appear in "safe areas" and it was hard to predict when he would show up. He would chase you through several areas before finally disappearing. That brings me to my next point.
Have enemies chase you through multiple areas. I personally find it hard to believe that most enemies stop chasing you after you went through a door, gate or anything. Nemesis in Resident Evil 3 did this as well. Enemies that really want you dead won't stop for something trivial like most doors. If you're stopped being chased, make it likely and it wouldn't hurt to have your enemy attempting to get through the door. (For example, you hear something bash against a heavy metal door after you just closed it behind you)
Make enemies somewhat smart. If forementioned enemy gets stuck behind an obstacle between you and it, have it find an alternative route. (For example, the enemy mentioned above realises it can't find a way into the factory you just entered. Instead, it will move to a window next to a corridor you already cleared/passed earlier.) Another example would be that if you take refuge in a wooden building and the enemy spots you, they will attempt to set fire to the building and will wait until you come out.
Don't have enemies hold human weapons, including zombies. Guns should be really limited in horror games anyway.
The protagonist(s) will take responsible choices at that particular moment. If I can't stand anything in a horror game/movie, its retarded protagonists.
I personally like the appeal that eventually you will have to build your own weapons. I don't believe that people leave ammunition all over the place; I really don't believe that enemies drop it, especially if they don't use it. In an eventual real horror scenario, you're probably down to improvising anyway, like kitchen knifes on broomsticks or something.
Avoid horror clishés or at least give them their own twist. For example, there is a shadow of a large tree in one of the windows and some twigs are pushing against the window. After a trigger, have a branch of the tree crash through the window because of a lighting hit or strong winds. Maybe part of the ceiling will collapse because of it, or have enemies enter through the broken window later.
In a dire situation, we should have multiple ways to cope with the situation as best as possible, but we can't tell for sure what will work best, except by trying it out or having slight hints on what would best. For example, you're being sieged inside a building. Outside, an enemy/mob is trying to get in. There is furniture and material to build a barricade. After you're finished building the barricade, you have about a minute to explore the building and take the right decision. If you can't decide before that, the enemy gets in and kills you. There is a wooden hatch below the carpet which leads to a ... basement? It sure is dark down there. You can close the hatch above you, but you need a source of light and the enemy will see that you went there. You can go upstairs. There is a fire escape, but it doesn't seem safe and you might need a key once you're downstairs since a fence closes off the area. You can also try the gap in the ceiling that will lead you to the roof. There are buildings adjacent to this one, but the gap between the buildings is 6 meters? will you be able to jump it? Will you need something else?
Have short flashes of the enemy's point of view. We see something stare at us from the darkness, trying to get closer and closer unnoticed.
Use all directions; In most games, most enemies come from your sides, behind you or in front of you. Enemies that come from below appear at times, but I really think that not a lot of enemies come from above in horror games. I think its a shame, because its not typical human to look up in real life. Force us to do something that we aren't used to. For example again, you're walking in a dark forest. Behind you, you're chased but the enemies aren't close for now, but you know they are still trying to find you; You're a bit in a hurry. Its windy and the trees are tall. You look up and can't say for sure whether you saw movement in a large tree further in front of you. Will you look for an alternative route? Can you afford to lose valuable time and will you risk to run into new dangers? Will you get nervous and fire at the tree? You might kill the enemy that was lurking in the tree, but the noise will know the enemies behind you where you are. Also, it might attract new ones. Besides, you're a bit low on ammunition and its dark. Will you ignore the sign? It might be just the wind after all, or maybe you can sneak past it. Did it already notice you? If not, you can pass this danger unarmed and without a fight, if it did, it will leap on top of you.
Do NOT include sidequests. Sidequests are luxury objectives. In a real survival situation, you shouldn't have the time, mind and material for those.
I personally do not mind the idea that trying to assemble weapons or other valuable items takes time. It might also make a lot of noise, so in a "real" situation, it will attract enemies. This will force the player to go into new areas well prepared, it will confront the player with new choices, or it will force the player to make due with the situation if he is out of silver bullets for example. Another example: Its a werewolf game. Its close to sunset and you're located in a desolate mountain town. You only have a few silver bullets left. Just now, you got your hands on material to make new silver bullets, but it will be night soon. Tomorrow, you planned to try to find a way through the everdark forest. In order to do that, you need to stock up. However, you'll need to go to the local restaurant (Large stone oven) to make new bullets. The restaurant isn't very safe compared to your where you are now; you don't want to be caught there. Smoke will come out of the chimney which will alert enemies. However, preparing bullets takes hours (will not be shown in the game of course) and will prevent you from leaving town early in the morning. It might be 2.00 PM before you'd be set to go, which might be way too late. You don't have that much food however and it might be full moon soon!
Just some ideas that would at least really get me in a horror game.
