Setting as Character

Horror, as a genre, isn't just about slashers and ghosts and monsters lurking in the shadows. One trope that offers endless possibilities is the idea of the setting itself being the antagonist.

The first reason I think this trope is so full of potential is that it throws focus back onto the characters. Characters who are fighting or escaping a defined threat can get away with relatively little psychological development, since their whole attention within the story is devoted to holding off the zombies or figuring out who the slasher is or exorcising the possessed person. Obviously the characters' personality and history can be a plot driver, but there is less scope to excavate the characters' inner lives when the zombie hordes are battering down the door. When the setting causes the situation, there is nothing to fight, and how the characters cope internally with the diffuse threat-- and cope with each other-- goes front and centre. 
Second, there are infinite options for how to develop the setting itself. It can be an antagonist by dint of being hostile to human life. For example, a group of travellers stranded in the desert or trapped by an ice storm and facing dehydration or hypothermia or starvation can set the stage for a horror story as the increasingly desperate characters turn on each other. Or the setting could be actively dangerous: an almost light-less jungle filled with predators, poisonous plants, and sinkholes is a terrifying place for characters who don't know how to avoid its hazards. 
Alternately, the setting can be a sentient place, a malevolent entity that seeks to destroy the characters in body or mind. The house in The Haunting, or the Overlook Hotel in The Shining are both excellent examples, but the location could be absolutely anything. If you go this route, the sky is the limit.

Using the setting as a plot tool is a wonderful way to create a horror story (or a story in any genre) that breaks out of genre tropes and surprises your audience.