I think the horror in Rosemary's Baby relies heavily on that brilliantly creepy apartment building. It's cruel, suffocating, mocking, grotesque - and I often get that feeling from seventies movies (eventhough RB is from 1968). It's something about the colours and...something else, it's almost a physical experience. Sometimes the best horror comes from the victim, not the monster. The monster attacks, you already know that. But if they can translate the fear, the incomprehension and the hysteria to the screen, then you have a great thriller (e.g. The Wicker Man). Unfortunately, the slashers (and most notable the blatant torture porn like SAW) confused fear with suffering. But I can't feel the knife, or axe or whatever. So it actually takes me out of the moment, I'm becoming a spectator. And then it's getting mind-numbing pretty fast. That said, the tree-rape in Evil Dead was quite a thing to watch. But then again, I don't think people watch slasher movies to feel frightened. Maybe it's more about "haha, that's nothing happening to me", kinda how you survive a rollercoaster ride. It's just the fun and the thrill.