This seems to be judged(as far as the crowed are concerned), entirely on how much it just cost the guy to throw his poor bike down a hill. They are all mad but it does look entertaining
the bikes and their riders are the stars of course but the incredible coverage by the cameraman/men is what makes it so watchable.
why not do it on a trials bike? this is the section next to my house. its a lot steeper than it looks.
Thanks for the hillclimbing suggestion @Chris , but I think I'll settle for Prescott & Shelsley Walsh.