A passage that I thought was interesting:

If you can keep the reader/learner/user in a flow state--where the rest of the world drops away--they'll love you. Seriously. And they'll want their friends and family to love you as much as they do. The flow experience is one of the most enjoyable and enriching times in most people's lives, and we all have the chance to help give our users just a little more of this.

Kathy Sierra's Blog

I think this relates to design, because design needs to flow well. A product has to be easy to use and to understand. If you have a product like that, many people will enjoy using your product.