So, my dear anon, what makes a good horror movie?
My own top three consists of 28 Days Later, Alien and the first Evil Dead.
So what can we take away from that?
Suspense, Tension and a good soundtrack.
28 Days Later (and I’m spoiling something here) has that wonderful scene with this track “In The House, In A Heartbeat.” The main character sets all hell loose, you can’t actually hear 90% of the action in the scene, just a steadily increasing crescendo as 95% of the human cast dies. And just when everything seems awesome? The film throws you a curve ball.
What can I say about Alien? Let me refer you here. That scene was shot more than 30 years ago. It's still filled with tension and suspense.
The original Evil Dead? Hmm, can I find the ending on youtube? I can in fact. So here it is. The Ash you see there has just butchered his sister, his girlfriend and two of his closest friends (I may be misremembering) and then they do THAT? Ouch. Kick in the crotch for Ash there. But that's pretty standard fare. That's a decent shock.
The problem I find with a lot of recent horror is that it relies a lot on shock for all the scares. Sure, something poping out at you is going to make you jump. Don't even get me started on this 3D shit.
A decent soundtrack will go a long way to building suspense and can add to tension. Psycho? The Shower Scene? Jurassic Park (and if you watched Jurassic Park as a kid, raptors may still terrify you to this day.)
Suspense and tension will leave a lasting scare that will keep you checking your closet and make you want to turn the lights on when you get up to get a drink at two in the morning. Not making you jump, and certainly not gore.