To me, the value of TED is "learning directly from the source".
Presentations about how an individual made the Technology or Design that is an integral part of our world today, or else a vision for a path to the future (also coming directly from the mouth of an individual who played a major role in shaping today's world). Or else they're entertaining, though I have always found the "E" component to take a back seat to the "T" and "D".
All of that, in a condensed format, and a never-before-seen presentation (i.e. not the same talk someone has been giving all across the country on a book tour).
The part about each speaker being the PERSONAL source of innovation is vital. Not someone who talks about other peoples' work. Definitely not someone whose presentation sounds like "gee, the world would be better if only lots of other people would do ________." (Unless, perhaps, they've already gotten tons of people to take that action.)
Mr. Bratton's blog post seems, to me, irrelevant.
He's correct that inarticulate presentations don't do well; this is nothing new. He may even be correct that TED has "raised the bar." But the purpose of the Conference (to my mind) is to foster a conversation among innovators, the creative and successful "movers and shakers." It's a discussion, not a promise to deliver on a platter any particular "future that hasn't come true yet."
For TEDx organizers, I'd say the priorities for avoiding the pitfalls he bemoans are: (a) Book speakers who have personally created Technologies or Entertainment or Designs that have already affected many people, who are willing to give a never-before-seen presentation, and (b) if possible, restrict the audience to individuals who themselves can contribute to the dialog in meaningful ways — ideally, successful leaders in their communities.