It is obvious that Brown comes from a design tradition that is usually described as the art & design school tradition, that is, it is the understanding and process of design as it is taught within traditional design fields, such as product and industrial design, interior design, fashion design, etc. Even though he comes from this tradition, Brown is joking about it by calling the people within this tradition as the "priesthood" of design.
Brown is proposing that we should move towards "design thinking". I have no problem with that, I truly support that idea. But what I don't really like with the presentation is that Brown makes this proposition as if it is something new and something that has not been understood until now. This is not at all the case.
The notion of design (or designerly) thinking has been around for a long time, and the ideas and ways of understanding design that Brown proposes have for quite some time been developed in an elaborate and robust way by such thinkers as Nigel Cross, Donald Schön, Bryan Lawson, Kees Dorst, Klaus Krippendorff, Harold Nelson, and others. The systemic perspective of design, that Brown also mentions, has also been developed by for instance C. West Churchman and Harold Nelson.
It is also the case that the ideas on participative design that Brown mentions as a new development have been around for quite some time. The basic philosophy and methodology of participatory design was explicitly developed in Scandinavia in the 70s and has grown since then and the international Participatory Design Conference has been around for several decades. Most of the ideas and issues that Brown mentions around participation have been developed both pragmatically and theoretically over the years, but not, however, by people from belonging to the "priesthood" of design.
My point is not to critique Brown in particular, instead I see this as a sign of something more interesting. The developments of design thinking that I mentioned above have almost all been done within fields not traditionally identified as design disciplines, and not part of the "priesthood". For most of the design thinkers I mentioned above, design has never been about "decorations" or "small design" (with Browns vocabulary). The theoretical development when it comes to design thinking is today moving faster than ever. There are more people involved, coming from more diverse disciplines (many not traditionally seen as design) making great contributions to, not only the understanding of design, but to the practice of design.
So, it is not within the traditional design disciplines that we can see the most interesting theoretical and practical advancements of design today. It is neither in highly "disciplinary" academic fields, that is, fields that are protective of the way they do things, and feel threatened by the development of design. The most interesting advancements of design seems to happen in highly transdisciplinary fields, that is, fields that work on real world problems that are overwhelmingly complex and messy. In these fields the design thinking approach, as Brown describes it, is the only possible way to successful intentional change.
The view of design thinking that Brown describes and advocates is the way to go. The good thing is that there are a lot of work already done when it comes to formulating such an approach, both theoretically and practically. But, even better, there is a lot of work remaining :-)