The community models that the authors propose are all voluntary virtual societies that are operate mainly as fun and entertainment outlets. The vast majority of those who participate and comment do not have a direct and vested interest in promoting certain ideas, approaches and solutions. The most they get out of it are bragging rights. (Lest not forget the very high signal/noise ratios in such public commenting)
Contrast that to the scientific world, where very large funds, influence and reputations are at stake. Add to this the anonymity that the Internet offers and one ends up with the impossible task of keeping opinions honest.
Digg and Slashdot etc. exist because the people who participate in these discussions do not have other ways to contact each other and make their opinions known. Scientists do have (and always had) far better channels: journals, conferences, meetings ... and frankly citing a paper is much more meaningful characterization of the quality of it that any number of public commenting.