As designers there has to be some ways you've approached establishing the 'fun-factor' in your games. You've likely done something in one game that may very well transfer into the future games you've worked on. You've likely played a game that did something really fun and you noticed that formula being applied to other games.
What are some ways to improve the fun-factor in game design? What makes Zynga games on Facebook so popular? Do rewards lead to more fun? Does failure lead to more fun? Does making a player come back in 2 hours lead to more fun? Have you read any good quotes or thoughts that really sticks in your mind?
There could be a good list of things to think about when approaching fun.