Eons ago, maybe eight years back, Slashdot was required reading for geeks. The commentary wasn't bad, the closest thing we had to the primordial pre-spam days of usenet newsgroups or the pre-Internet BYTE discussion forums.
I still read Slashdot, albeit once very few days. The articles are quite good. The commentary, however, is almost worthless.
On the other hand, the tech blogs I follow are terrific. The information flow is very high quality -- as good as anything. Particularly from single-author blogs.
I don't know the entire story about why Slashdot failed -- except I haven't seen significantly better quality discussions anywhere else. Single voices, for me, are better commentary and analysis sources than communities.
One clue might be that the comments I wrote for Slashdot were never very highly rated. Perhaps they were simply uninteresting, but the same fate seemed to apply to all comments not posted within a few hours of a news item. Slashdot effectively rewarded speed over all other measures.
I don't think an unfortunate rating system was the whole story however. I suspect that personal ownership (editorial control, strong identity tie) of one's commentary stream and extended memory will always be a key ingredient of any future system -- including future systems of federated journalism. Blogs allow that, community forums don't.