Clowns are one of those things where no one was scared of them until some magical switch was flipped and nearly everyone recognized them for the creepy bastards they are. But what was the switch? What turned them from fun-loving entertainment for children to sinister, maniacal embodiments of evil?
I never liked anything in a mask. I don't trust people who feel the need to hide their face. Combine that with a painted on smile and constant laughter and the creep factor goes through the roof. FACT: No one who is up to any sort of good is that happy, ever.
However, I think for many Poltergeist had a huge influence on the clown hatred of Gen X and Y. Who can forget that scene of the clown doll, who was just creepy throughout the movie, pulling the kid under the bed. Not okay, Steven Spielberg. Stephen King's It wasn't too bad in the fear factor either (well up until the ending it was solid at least) and Timmy Curry completely sold Pennywise the clown as the stuff nightmares are made of. And let us not forget the real life killer clown, Pogo, or more commonly known John Wayne Gacy.
Honestly, it is a wonder clowns remained as innocuous as they were for as long as they did.