It seems that what is effective in horror films is extremely subjective. What is truly frightening/effective for one individual is laughable to the next. To me,
Halloween is the film epitome of the idea that something is lurking in the dark, waiting to spring out and harm you. I'll grant that my view may be skewed a bit by experience:
Halloween was one of the first horror films that I experienced as a young person, and it scared the liver out of me the first time I saw it. At that time, it was really pioneering. Subsequent "slasher" movies have done nothing but make me yawn...I've never cared for them (with the possible exception of the
Nightmare on Elm Street movies, but those more for Freddy's droll humor than any chill factor). But in the original
Halloween, it seemed that Carpenter really captured the essence of some terrible presence hidden in the shadows. For some reason, I found the original
Alien effective in a similar way.
Still, I tend to find myself most appreciating horror movies that play on the imagination, rather than expicit blood and gore. Although it's a movie that people either absolutely loved or hated, for me
The Blair Witch Project was pretty frightening the first trip through. Proof, for me anyway, that what you
don't see onscreen is often far more frightening than what you do. The most recent horror film I've seen was
White Noise, a movie I really enjoyed until the last fifteen minutes. Without detailing the ending (for those who haven't seen it), for me the whole movie curved off in the wrong direction in its final sequence. The subtle terror established in earlier in the film was quite good.
Side note (and not an attempt to pull this thread off-topic, I promise): For fans of Myst-style adventure games,
Dark Fall: The Journal is a game that can really send chills up your spine. If you like subtle scares and exploring haunted places, it's well worth playing.