The thing that made Psycho so creepy was the character himself. It was a horror movie that didn't have to rely on massive amounts of gore to be scary.
That, and back then, that kind of horror was unheard of. People started having nightmares about killers coming to their showers, and people left the theaters with uneasy stomachs. By today's slasher standards, it's nothing more then creepy, and a little silly.