You raise a few interesting points. First, I would just note that your major premise is one that we've discussed a few times at Boys' events, although I can't recall whether you've been present for any of those discussions. It was the subject of a short but prescient book by Cass Sunstein titled Republic.com. He argued, precisely as you have, that proliferation in media sources will fuel self-selection of sources that confirm preexisting biases, undermining our cultural common ground and fragmenting our social beliefs. I think the threat is real, but can be overstated (as I'll discuss further below).
Second, I know exactly what you're talking about with regards to the comment sections on news sites. That stuff is purely toxic. It's even worse than you find on the purely partisan blogs and forums (e.g. DailyKos or RedState). News sites and YouTube comment threads and things of that nature appear to draw the absolute dregs for commenters. It's disturbing to read it, but ultimately I don't think that amounts to much. I suspect that it really is a very small percentage of the people who read the news stories or watch videos who post those comments (and probably with a high percentage of repeat players from one site to another). The numbers of people who participate in sites like DailyKos and RedState are more significant. Of course, those people also tend to suffer from groupthink and hyperbole and certainly represent a fragmentation in worldviews. But again, I'm not yet convinced that this is especially problematic. Their influence is ultimately fairly limited, as each party's Presidential primary this year demonstrated. In both cases the netroots favorites (Edwards for the Democrats, anybody-but-McCain (Ron Paul?) for the Republicans) failed to win.
Ultimately I think the democratization of public discourse via the Internet has been a tremendous win for the public. Contrary to your contention that we've seen a disappearance of sincere public discourse, I think we're seeing the emergence of new and important avenues for public discourse. There is a dialogue taking place on a community of blogs that surpasses in intelligence and candidness anything I can recall from the pre-Internet (or early Internet) days. To name just a few: Andrew Sullivan, Obsidian Wings, Matt Yglesias, Nate Silver, Al Giordano, Dan Drezner, Greg Djerejian, Ezra Klein, Tyler Cowen, John Cole, Cunning Realist, Josh Marshall. This short list covers a pretty broad spectrum of opinions (and certainly there are many others out there). These people are intelligent, generally well-educated, have diverse areas of expertise, and are interested in honest and sincere dialogue. Better yet, many of them read one another (and many other bloggers) and actually engage in back-and-forth discussion.
While the readership of the blogging community is still not at the same level as traditional media consumption, I think its influence is considerable. The bigger blogs (Sullivan, Marshall) draw in the neighborhood of a million page views a day, which is not insignificant. Moreover, that readership is disproportionately drawn from politicians (or at least political staffers), policymakers, and the media. This blogging community has the ability to affect campaign narratives. I think this is one of the reasons the McCain campaign flailed so wildly from one narrative to another over the Summer and Fall. The McCain strategists, all Karl Rove proteges, couldn't understand why their Obama narratives were not sticking the way the Al Gore - fibber and John Kerry - aloof liberal narratives stuck. The bloggers ripped their idiotic narratives to shreds and were quick to pounce if the traditional media went too far astray.
The emergence of this community presents a challenge to Sunstein's predictions. Certainly there is plenty of evidence on the Internet of Sunstinian self-selection and reinforcement of biases. But there is also a conscious acknowledgment among others that in order to be relevant as a thinker and a political writer one must be willing to inhabit the same world as other writers and one must be willing to engage differing views on more than a superficial level. Certainly this creates some tension between the self-reinforcing Freepers and Kossacks and the reality-based bloggers. On the conservative side this tension has spilled over into something like open warfare with the Freepers and Redstaters attempting to expel any conservatives who do not fully acquiesce in the Bush/Rove Republican orthodoxy. It's not clear how this is going to play out, although it seems inevitable that it will end badly for the Freepers, who are at this point quite remote from reality.
To conclude, I think, Henry, that you and Sunstein both had useful insights into the impact of hugely diverse media landscape, but I think the danger here is easily overstated. The broad expansion of the media landscape has occurred because the barriers to publication have dropped to almost nil (case in point: the Boys Weekend Journal), and the elimination of these barriers has also had tremendously positive effects. It does require people to be more savvy in their consumption of, and interaction with, the news, but the Internet has also made a great many people far more savvy about news media than the public has previously been. And the ease of entry into the market and the ease with which consumers can move from one product to another has allowed for something much closer to a true meritocracy than has ever existed in the traditional media. Nate Silver could never have gone from obscure bystander to political media powerhouse in less than a year in the old media market. I would go so far as to say we are entering a renaissance of political discourse. But the interplay between substantial number of people who have defied Sunstein's expectations with the still considerable masses who have conformed to his expectations does warrant continued observation. While relations between the two groups may frequently be strained, I think by virtue of pure merit alone the reality-based bloggers will always remain relevant and important.