Stab wounds aren't all that messy
If you believe all those other silly horror movies, you'd think that stabbing someone would cause copious amounts of blood to fly all over the place and seep into a big pool on the floor. Prom Night showed me that you can stab someone in the torso upwards of a dozen times, and they will bleed only enough to stain their clothes. This makes hiding bodies quick and effortless. No fuss, no muss.
Dead bodies retain the ability to spontaneously jump out at people
Proof of life after death? Perhaps. The fact is, corpses retain some sort of intelligence that allows them to leap from their hiding places at just the right moment. And it's not just gravity. Judging by the one that busts out of a cabinet in Prom Night, there's some real muscle power behind those leaping dead guys.
Dudes in baseball caps are scary as Hell
And you thought a mask made out of a tanned human face was creepy. Prom Night has done for baseball caps what Friday the 13th did for hockey masks. Seriously, I bet you can't watch a hockey game without the lingering suspicion that the goalie is a homicidal maniac. Now baseball is ruined too. Or even walking down the street in Helena, Montana.
Being psychotic imbues people with super powers
There's a scene in Prom Night where blood actually does spill beyond someone's shirt. A few drops fall onto a bed from the air duct in the ceiling, and the homicide detective orders his men to bring him a ladder to check it out. Why didn't the killer need a ladder? Because he can fly, of course. Apparently he can also vanish and appear at will, transport full-grown human bodies effortlessly, and make no sound when moving, opening doors, and murdering people.
Boredom equals suspense
Much like you can substitute shortening for butter in a cookie recipe, in a horror movie you can make up for a lack of suspense with the liberal application of boredom. In a suspenseful scene, the characters are basically just waiting for something to happen, right? So why not just throw in a bunch of scenes where people are waiting for something to happen and call it good?
Unrated is not the same as NC-17
I've never claimed to understand the ways of the ratings board, but the unrated edition of Prom Night strikes me as solidly PG-13. But "Unrated" just sounds so edgy.