This talk was giving publicly at Qcon SF 2015. It goes into the decisions made when designing Yelp's PaaS: PaaSTA: http://paasta.readthedocs.org/en/latest/about/paasta_principles.html
The outcome of these opinions and the effort of integrating the components mentioned in the talk is PaaSTA: https://github.com/Yelp/paasta
The purpose of the talk is to inspire others to think about their ideal PaaS, and how they would build it, and perhaps draw upon some of the principles/opinions used at Yelp.