Is it the boo? Or the anticipation of the boo?
I see questions all the time about what scares people... what creates tension... is comedy ok mixed with horror?
I think the answers are dependent on the audience. Horror has as many subgenres as Ben and Jerry have ice cream.
Some like blood, some paranormal. Some just go for the boobs. I think that creating that believable unknown, the story that sucks you in and endears you to a character you can relate with, is the most successful. Then that character's woes become your woes. Their fears your fears. When you're teetering on the literal edge of your seat and something as trivial as a tap on the shoulder sends you to the ceiling, they've won. That's a success.
There's also the mood factor. Something so eerie that it doesn't get you right away, but hours, days, or weeks later you get a chill down your spine that you can attribute to the book or film--success.
Flimsy characters make for bad story. If I'm rooting for the bad guy to take out the boring or annoying goons then it's a fail. Maybe fun while it lasts, but forgettable as soon as the popcorn is gone.
What do you think? What makes a success? It doesn't have to be horror. I think the rules of eliciting an emotional response from your audience cross all borders.