Law of conservation of complexity

> The further down in the software hierarchy that you push the complexity, the less work has to be done by everybody above.

This. From the interview[1]. I think this is the most important takeaway for people building software. An inherent assumption here is that the deeper layers in the software undergo less changes and iterations compared to the outer layers. This may not always be the case, but it often is.

1. http://www.designingforinteraction.com/tesler.html