In a polar opposite of the recent PopCap talk, Teut Weidemann, lead designer of Settlers Online for Ubisoft's Blue Byte studio, says that the key to free-to-play success is to exploit human weakness. Essentially, find those areas in which players can be monetized, and go after that aggressively.
See the entire article at the link below.
I found this to be very interesting.
http://www.gamasutra.com/view/news/2...knesses.php%5D