Whenever I see something that really bugs me in a movie I try to make a mental note of it and why it bugs me. This is just a list of some things I've thought of while watching many many horror films. The should be obvious but they apparently are not. I'll be adding things as I think of them.
Fingerwitch's Rules of Horror Filmmaking
#1 Always Use Good Lighting
I see so many movies that seem to think that, in order for a scene to be scary, it has to be really fucking dark. It's not scary. It's annoying. And probably a waste of money. Why pay actors and crew to shoot a really dark scene? Furthermore, why would I want to watch that? I can cover my head with a paper bag and run around screaming for free.
#2 Never Plan for a Sequel
Leaving the plot unresolved at the end of a movie is a surefire way to piss everybody off. They will get pissed off at your stupid movie and it's stupid ending and steal it out of spite. Just end the fucking movie. If you make a shitload of money, cash in on the sequel. If not, do something else you lazy fuck. The worst thing is if the movie is really good, that will leave me feeling horribly unsatisfied.
#3 Keep Explanatory Monologues to a Minimum
If you need to write three pages of the villain explaining their complex and nefarious plan to make the movie make sense, then your plot is way too complicated. Keep the explanation of the plot to under a minute of running time.
#4 Cast the Right Accent for the Job
And if you really can't find any English people for your movie, at least cast an American who can speak with a decent English accent. This rule applies for any combination of nationalities.
#5 Don't Introduce a Character Late in the Film
I would say probably the sixty minute mark is the absolute latest you can successfully introduce a major character. Especially the killer. Introducing the killer during the last fifteen minutes is a fucking cop out.