A few things that stood out to me in Horror that make them to be in that genre is the shadows. If there isn't enough shadows in the film this it's not really a horror film. Second, is the dead silence in a few parts that you can tell something that is going to catch you by surprise. Creepy music that sounds twisted and screwed to not sound like it would be on someones playlist unless but if it is you should check that person out. Dutch angles are a big thing in Horror films.