How can you make a user experience addictive?
dfield: Last summer, I had the privilege of attending a talk by Mark Pincus at Startup2Startup.  One of the tips he gave that really resonated with me was to "gamify everything."For example, if users have to enter information to make your service more usable, give them a progress bar.  In general, let the user "grind" - give them small, repetitive, mindless actions that are easy to accomplish and give instant, positive feedback.  Above all, make the experience simplistic enough that they know what to focus on but interesting enough that they don't leave.Reputation systems that have an effect in a community can also make people come back.  Just look at HackerNews ;)