Horror Movies are generally known to be so bad it almost constitute its own separate B-category. It is true the acting is sub-standard, the lighting is purposefully disturbing, story lines follow a predictable template and the dialog is cheesy. Yet, there are some things they get almost consistently right, at least as entertainment vehicles. These are things I often find lacking in artsy films and even mainstream ones.
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The most important is the pacing. Horror movies always move along at a good clip and keep the tension throughout. They are also generally short, where many of the more serious kind drag on way too long, the producer too fond of his own work to cut unneeded scenes.
Horror movies establish the conflict up front, often with an introductory scene where we see the first victim getting it. Not even in the worst C horror movies have you any doubt who is going to be after the protagonist. (The protagonist is often established later, because in horror, the monster is really the main character, even though POV typically follows someone else).
To stay with the conflict theme, in horror, the fight is not only between the monster and the people, but always between the people themselves. In the best horror movies, a group is trapped somewhere, whether on a spaceship, on an antarctic ice-shelf, or an old house. This is a terrific concept for drama.
Unlike many pretentious 'films' made by pretentious movie-makers, horror flicks do not take themselves seriously, even making fun of themselves. I usually get at least one good laugh out of them, something I do not always get from comedies.