Let's talk about the problem of running horror games.
Last year I went through a phase of reading almost exclusively horror - I was ploughing my way through Lovecraft, Ramsey Campbell, and Thomas Ligotti stories faster than you can say "the blind, voiceless, tenebrous, mindless Other gods whose soul and messenger is the crawling chaos Nyarlathotep". I naturally started thinking about running Call of Cthulhu again, and even bought the 6th edition core rules.
But I also started thinking about how difficult it is to run an actual horror game. I use that wording advisedly, as a way of distinguishing it from games with the furniture of the horror genre. These are not difficult to run: it isn't hard to run Call of Cthulhu or Unknown Armies, especially if you are making things a little bit rail-roady, but also if you are willing to set up a more sandboxy investigative type game.
However, having the furniture of horror is not the same thing as horror itself, and there are elements of the horror genre that I think are not only nearly impossible to emulate in a traditional game, but actually militate against it. Proper horror, in my view, revolves around two things:
1. Powerlessness of the protagonist. While a protagonist should be active, and usually is, ultimately his fate should be out of his hands. That's what makes it scary. It may simply be that the protagonist is unable to curb his own curiosity. But still, horror seems crucially to be, to some degree, about DEPROTAGONIZATION. And I don't mean whiny "I'm not allowed to be awesome, boo-hoo" style deprotagonization. I mean the GM mandating that unutterably shit things happen to your character for no other reason that "It's horror and your fate is out of your hands".
2. Bleakness. I'm not a believer in the hero or heroine triumphing over evil in a horror story. Even if he survives, he should be physically or mentally ruined. Ramsey Campbell is the master of coming up with endings which imply that some unfortunate soul will be in torment forever, and Ligotti is expert at implying meaninglessness and complete lack of hope, but more mainstream horror can be just as pessimistic - in most of Stephen King's books there is a genuine streak of sheer nastiness in the endings, giving the sense that everything has changed for the worst. But this, in a traditional game, would be equally as DEPROTAGONIZING; generally, you want your character to have goals, to improve, and raise his or her standing. Not face ever more hopeless situations unto gruesome death or eternal suffering.
That's not to say you can't have powerlessness and bleakness in short bursts. Murderous Ghosts manages this - it lasts an hour, terrible unspeakable things happen, and then it ends. And you don't care because it's an hour and it was fun. But it isn't the stuff campaigns are made of.