TLDR. Well, not so short description, but I think this is the gist of the talk. This is a very thoughtful talk, from the creator of one of the most active open source projects today: Drupal. Love it or hate it, Drupal attracts a very big community, and in growing it has also accumulated a lot of complexity that it is now a challenge to contribute. Dries compares Drupal and other open source software projects to "public good" following very closely the Wikipedia article about it, and using that analogy tries to map some of the solutions found to take care of public good and apply them to Drupal. I'm not 100% in agreement with Dries proposed solution, but the talk is very interesting.
