Silent Hill: Downpour Features

What separates the good horror films from the really good ones? What holds the genuine shockers apart from the moderate spine-tinglers? I've been thinking about this - for at least two minutes - and I'm pretty sure the answer isn't sharks. In fact, I'm pretty sure the answer is something like ambiguity: a creepy sense of uncertainty, a little bit of leeway for your imagination to work its dark magic.