I was thininkg about this last night and the only paysite I ever joined was siliconefree.com waaay back I think in 1999/2000. At the time I was broke and in college.
Why?
Unique and hard to find content (if you like massive tits)
Decent price point 19.95 actually I think it was 12.90 or soemthing back then.
Simple design - the design was very basic, no bells and whistles and the content sold me, not the fancy text effects.
The kicker - a completly transparent tour page...surfers where allowed to view and browse the entire members area, viewing only thumbnails that linked to a join page.
I remember staying a member for a few months i belive. I think more sites that have transparency to thier tours would increase sales. We played around with a "look inside" feature on some of our sites they seem to help with conversions.
Just my two cents from my experience as a former broke horny surfer...
