There are pretty much two kinds of horror films: ones that build up to something and ones that don't. Usually, you know when you're getting into a film that isn't building up to deliver something special at the end. That something special is usually a resolution to ongoing suspense, and at its best, it's a thought-provoking discovery.
There are tons of films that resolve the suspense with a finale worthy of the buildup. My all-time favorite film, Wait Until Dark comes to mind, or Psycho's twist ending. Others leave you with something to carry home with you, like where American Psycho builds up to its most interpretable material, or Martyers, which concludes with a philosophical rumination trapped somewhere between the Cartesian and the Lovecraftian. Then there are those films that don't resolve in the most memorable of ways. Most slasher fare is very formulaic, and you can usually tell by the box it came in what you're in for. These films aren't all bad, by any means. But instead of getting engrossed in the story, you can appreciate the film for the special moments it dishes out, like a great jump scene, a suspenseful buildup, or something you've never encountered before.
The Texas Chain Saw Massacre is one such film, which established the slasher formula that we've learned to anticipate. The pacing and presentation of the film engrosses you, so that even today, I can still find people who don't know the film and say, "you have to see this!" The Shining is another presentation-driven film. The plot is fairly static, and the characters sort of act as outlets for the hotel's hauntings. When the drama is over, you don't really notice that the characters have left the hotel; the movie wasn't really about them anyway. It was about what that creepy July 4th photograph represents, which is enduring spookiness, plain and simple.
I really resent a film that tries to build up to something special and then tanks at the end. It prevents me from appreciating the positive qualities of the film. One way to do this is to give a twist ending that isn't logical (High Tension...). Hide and Seek does the same thing, where I feel let down by the twist, not because its illogical in this case, but because the only thing the movie had going for it was its effort to take me by surprise, which it totally failed to do.
Two of my top worst let-down endings are in Frailty and The Strangers. Frailty had me riveted through the whole film. I think that reductio ad absurdum is no way to hold a real debate about religious fundamentalism, but it is a perfect method to devise a quality horror plot. But at about the last possible moment, we lose the whole stream into an absurd twist, the motivation of which I simply cannot discern.
Similarly, I was thrilled by The Strangers, right up through "because you were home." Closing on such brilliantly nihilistic dialogue whould have been stellar, in my opinion, but then we're subjected to a few more minutes of aimless cruelty and the obvious preparatory moves to allow for a sequel. Ambrose Bierce wrote that the definition of a novel is "a short story padded." Already brief, The Strangers could be tighter.
This is the end of this post. I think it had its moments, but I don't have anything special for a closer.