Simplicity through depth.
It's been my experience that complexity in interface design arises from a failure to synthesize the functional components into more straightforward models. I believe that by building a depth understanding of the problem, both human, technical and operational, it is possible to discover more fundamental and familiar forms.
For the natural language interface of Google Now, this meant understanding voice recognition, semantic parsing, linguistics, pragmatics and the structure of conversation. For the voice, video and instant messaging capabilities of Skype, it required a grounding in communication theory, the effects of computer mediated communication, media naturalness and network societies. For the early development of the iPlayer at the BBC it meant understanding the implications represented by the collision of linear broadcast media and on demand digital media.
As Einstein said, if you can't explain it simply, you don't understand it well enough.