Really, it's the unknown that drives the fear of anyone or anything. If you truly knew and understood a clown, you could stop being afraid, just as knowing what is about to happen would allow you to accept the event and react in a way to your best advantage.
The fear of the unknown can be triggered by any type of object or personification: symbols; clowns; blood; freaky ladies; etc.
That being said, if I were to suggest something for a horror game, I would incorporate as many phobias as possible into an atmosphere that genuinely makes it seem plausible that those triggers could all exist. Beyond that, I would ensure that the player is kept in as much suspense as to the nature of the game and the world as possible.
Good examples for comparisons, I think, are the Resident Evils and Silent Hills. When I played Resident Evil, I was occasionally treated to an actual scare, but mostly I was just focusing on navigating around and solving the puzzles. With Silent Hill, my pulse rate was elevated, I was breathing more frantically, and generally getting that stomach-knotting adrenaline feeling when you have a real fight-or-flight reaction.
It should go without saying that everything in this post is a reflection of my own opinions and observations and in no way do I presume to speak for everyone else.
