challenge
Despite having seen Kathy Sierra speak at GUADEC last year, her keynote at LCA was still engaging, entertaining and thought-provoking. One thing it tied nicely with was Rusty's talk about Wesnoth, in particular the part about stats.wesnoth.org, which collects information about players' progress through the game. Kathy talks about how important it is to get the amount of challenge in the things we make just right — the Goldilocks rule. If something is too easy, or too hard, people lose interest.
The Wesnoth statistics allow campaign developers to see when players give up on the game levels they write, or when they complete them too easily. This kind of feedback makes it much easier to get the difficulty just right. Perhaps we should be thinking about other feedback systems we can use to let us know when we're putting the wrong amount of challenge into our software.