If you're anything like me, you find clowns to be scary as hell. You also love being scared so you seek out those things that make you want to pee your pants. I know exactly when my clown fear kicked in. The year was 1990, and Stephen King's epic novel IT becomes an even more epic mini-series. I was 7 years old, and had a Stephen King fan for a mother, so yes, I was allowed to watch it. Needless to say for year I was convinced that Pennywise and Freddy lived in my closet.
I think the scariest thing about clowns is not that they are monsters, although Pennywise is, but just dudes in suits and make-up, so they are more reality based than say Wolfman.