I agree with Emmitt that the best thing about Psycho is how it manages to take only two killings and still make the film terrifying. Unlike today's slasher films where the audience can literally look at the characters and decide who and in what order they will die. Psycho is great because it put the audience into a situation that they more than likely engage in on a daily basis (showering) and makes them feel that vulnerability.
Any slasher film that preys on the idea of when the real life audience are the most vulnerable have a higher chance of actually being terrifying compared to those that continuously put a group of kids in the woods only to be chopped up by some deranged and child abused killer. When done the right way, a small scene that targets our most basic fears (killer under the bed, noises in the house)are the biggest nightmare to the audience because they start out with an element of the unknown. The fact that the first viewing audiences of Psycho did not know that Norman was both the killer and his mother was doubly terrifying because it forced them to deal with the idea that a normal perception of behavior could also serve as a phsychotic one. That we may never know the type of people we actually surround ourselves with. Those slasher films that can leave the audience still questioning after the film will always be transcendent of the rest.