1. A good (or at least understandable) storyline.
Of course. If we don't know what the hell is going on, we're not going to be interested. I tend to like romantic background stories involving the characters, or some other kind of sob story. It just makes me sympathize with them, and I want them to survive that much more. Plus, there's got to be a good story about the killer(s) as well. I also tend to like sad or traumatic story for them. I like that love/hate relationship I can feel for the killers.
2. Creative kills & lots of gore.
No one wants to see a bunch of people get stabbed in the gut. A knife will get boring after a while, so we need to have some creative and unique kills going on. I think A Nightmare on Elm Street is absolutely fantastic in this category. Since Freddy kills in peoples' dreams, there's literally nothing he can't do, and the resulting kills end up pretty fucking epic. Some people will say that the kills don't matter, and that the story is the most important thing, but that's a load of horseshit. The story is important, yes, but without those awesome kills why bother coming back for more? And yes, I'm a gore whore, so the bloodier and more vicious the better.
I realize not all horror relies on creepy lighting, but I love the effect it can have. Something popping out of shadows, or being barely visible, or maybe just slightly blurry. I also like the kind of dark lighting a lot of supernatural horror has. It just adds a little oompf to what is already a good movie.
Yes, yes, yes. Think about it. There's a woman hiding, the killer is close by, and about to sneak up on her. It's something very simple, and if it is completely silent, the tension will be gone. The suspenseful music just makes our hearts beat faster.
I think Asian horror is very good with all of this (except the titties). Their use of lighting is amazing, the storyline is always good and the villains (or ghosts, usually) are always sympathetic. And they're always just downright creepy as hell.
It really does matter where a movie takes place. Some places just aren't scary (even though I realize some movies actually succeed in making non-scary places scary). I always like woods, carnivals, and creepy old houses.
Yes, I said it. I know I'm a girl, but I grew up watching 80s slasher movies. I'm set in my ways, and I stand firm by my belief that if I don't see titties, it ain't a horror movie. Not literally, of course, but it just kind of makes everything better. Everybody loves some good T & A.
7. A good villain.
This might be the most important. I already said that the killer must have a good story, but there are other things that make a good killer. He (or she) should have some sort of thing. Like, he always chains his victims up, always takes their eyes as trophies, or always jacks off over his corpses. It doesn't matter. He needs some kind of constant thing. He also must be vicious, unstoppable, and just downright awesome. He doesn't have to be super strong, but if he's not he should be super smart. If he doesn't rely on his strength to get good kills, he should rely on his brains.
8. Good victims.
Some people might say, "Who cares about them, they're going to die anyway." But it's very important. They should be likeable and interesting. Otherwise, we won't give a shit about them and the whole movie will be ruined. I've watched several movies with completely dull, and the whole thing was ruined for me. So the other characters are very important.
9. Scary moments.
This one's simple. It's those oh shit moments, the ones that make you want to jump out of your seat or shit your pants. The moments that shock & awe you and leave you on the edge of your seat begging for more. I love it.
10. Sex, drugs, and rock and roll.
Most of my favorite movies include all of this. It's typical, but it works. There's usually a group of people getting fucked up, partying and getting it on. It's all fun and games and then BAM! A bunch of dead fuckers.
11. Comic relief.
I love when a horror movie has a certain character that is fucking hilarious. That way we can laugh, get scared for a minute, laugh again - at least until the funny-man is killed. Think of Ned in the first Friday the 13th. He was silly and made us laugh all the way until he was killed. In A Nightmare on Elm Street, the comic relief happened to be the killer, which gave us a strange but wonderful feeling. In The Evil Dead, it was the hero that was our funny-man, and we all severely love him for it. It's kind of like life. There's got to be some entertainment and some funny stuff amidst all the horror.