- Set the difficulty level correctly. In an app, it should be ridiculously easy to do stuff. In a game, the challenge should be scaled appropriately so the ga

- Set the difficulty level correctly. In an app, it should be ridiculously easy to do stuff. In a game, the challenge should be scaled appropriately so the game is neither boring nor frustrating, and so that players continue to develop and use new skills as the game goes on. - data, stats, and awards. People like to see how many posts they've made, how much karma they've accumulated, how many songs they've listened to, or how many zombies they've killed. They like to be given titles or trophies for passing milestones. - feedback from friends. Even better than getting that 10,000 zombie trophy or that new forum rank is getting congratulations from your friends on your accomplishment. - tickle the senses. Pretty graphics or sounds make for a neat experience in games or certain social apps. (The game "Auditorium", at playauditorium.com, is fantastic for this!) - replayability. The experience should remain fun time after time. In a game, the challenge should have some variability. In any app, the "rewards" -- karma, titles, pretty graphics, or what have you -- should update often enough that users are consistently reminded of them. For socially-focused sites, it should be easy for users to generate good content for each other so that each visit seems rewarding. (Example: Grandma finds it rewarding to see new pictures of my baby. Facebook makes it easy for me to upload new pictures, and easy for her to view them, so she gets new pictures fairly often. Therefore, Grandma is addicted.)