1. infinite ammo
2. cover-based shooting
3. regenerating health bars
4. lots of jump scares and loud noises
5. spooky monsters
I'd say the near opposite. Being defenseless is very important in actual horror, as it adds to the threat of what ever is trying to kill you. The most you should give the protagonist is either a mediocre pistol with little ammo, or a melee weapon. It should also be effective on very few monsters, as the game would turn from horror to FPS very quickly. Regenerating health is OK, but it should not be overpowered. And, personally, I think jump scares should be used sparingly, but I know not everyone likes subtle horror, so it's really up to you.