The key difference between a traditional academic lecture and a TED talk is the audience. Here, the TED audience functions as a cultural sieve. Yes, a "dumbing down," relatively speaking, but then again, lay audiences are so by definition; academic lectures are attended by self-selected experts in a given field. TED audiences, on the other hand, are usually a collection of diverse specialists and semi-specialists from different fields, and thus may be considered a lay audience for all intents and purposes. Moreover, the internet audience is almost entirely a lay audience.
The benefit of TED is, as others have stated, the "cross-pollination" of ideas that one would likely not have exposure to in other ways. And, unfortunately, that implies a lay audience, with all that it entails.
So, yes, that implies showmanship. It implies humor. It implies oratory skills. Because that's how you reach a lay audience.
Good presentation has much in common with good leadership skills. Per Howard Gardner, there are 3 key points that many good public speakers and leaders possess (http://ecglink.com/library/ps/stories.html):
1. Having a central story 2. Fitting the story to the audience 3. Data is not enough.
This is something that speakers like Gladwell get.
It also means that academic lecturers need to up their game. And this gives them a model for what a lay audience wants.
I once had a lecturer who was very proud to tell his postgrads "real science is boring". Yes, it often is. And for those of us who can stomach boring, it's a paradise of intellectual nourishment.
But most people aren't like that. Most people need shiny things to guide them.
Is this fundamentally a good thing? Probably not. But we don't have the luxury of pre-screening an audience through self-selection, as is the case with most academic audiences. An academic may be absolutely brilliant, but if he/she can't connect with the lay audience, it does no good for the ideas he/she espouses.
Just view the TED talk by Marvin Minsky to see a good academic flounder in front of a lay audience. http://www.ted.com/talks/marvin_minsky_on_health_and_the_hum...
Take home point: yes, this is a dumbing down. Take it for what it is. Instead of complaining that audiences don't react to dry academic lectures, academics need to study people like Malcolm Gladwell to emulate his delivery and rhetorical style, without sacrificing their academic integrity. Easier said than done, but there are plenty of substantive talks on TED to suggest it is possible. If indeed the medium is the message, then academics lecturing to lay audiences need to get the message.
P.S. For those worried about TED's influence on academia, methinks it will be minimal. Academia is a very conservative endeavor, and whereas sales-pitch talks (like TED) may get sexed up, academic lectures to academic peers will continue in their wonderfully "boring" approach, for many years to come (and thank goodness for that). Why? Because academics will never cease to try to poke holes in others' ideas. Intellectual snark works in academia. It doesn't work with a lay audience. However, narrative does.