Hardest and most rewarding was all the serious engineering we did to make the app and gestures fun and fast. Most of the components in the app are custom, from the navigation controller to the story menu down to the scroll physics. Aria just answered this Quora question that goes into some of the details.

http://www.quora.com/Prismatic/What-were-the-techniques-Pris...

If you have any particular things you'd like us to elaborate on, we'd be happy to answer your questions.