I agree about how you'll never be able to keep things secret in this day and age but if you really don't want to be spoiled then don't go to places where you may be spoiled, you know? Things don't change, even back then if you wanted to keep Psycho's ending a surprise then you didn't look for the book or avoided people who were talking about it. It may be a bit more difficult to hide from spoilers but essentially it's the same thing.
But back to Psycho - I think what makes it great is not so much the shock value - the amount of violence and sex (kind of anyway) - but how effectively Hitchcock manipulates the audience the whole time. First you think it's about an affair, then about how she steals the $40,000, then about whether she gets caught by the policeman, even at Bates Motel you wonder if she's going to repent after that talk with Norman, and then finally she dies and even then Hitchcock plays out the MacGuffin as long as he can. It's really awesome. And of course the final bit at the end, the last speech by Mother about how she'd "never hurt a fly" - that's the definition of creepiness.