Warning, this is all going to sound more pretentious than I am usually comfortable being:
It seems like this happens a lot, there is a sort of cycle, though I feel like it has changed and something new is forming. A lot of sites that start as reliable bastions of intellectualism or emerging cultures eventually go mainstream and slowly become diluted. TED hit this point probably about a year ago and quality will go downhill it seems. Similar things can be said for reddit, and other sites.
Perhaps this is a reflection on the ethos of the web 2.0. By being democratic, having content created flow from the bottom up, once a site becomes too bit, it will start playing to the common denominator.
Or perhaps I need to have more things to do at work and less time to reflect on the Internet.
PS. This TED talk is now officially meta.