It's true, Slashdot is a cesspool. It attracts a host of pathetic vermin, who apparently have nothing better to do than troll the place. What passes for analysis on Slashdot is at its very best mildly interesting and at its worse just highly attenuated groupthink. But, Slashdot thrives. There is a community there, it's undeniable. A lot of people read it, mention there is important in some circles and as an Internet Media, it's somewhat influential in that mention there can bring many many hits to your site and attention in other more traditional media.
Bruce Perens' Technocrat, which aims to be a more serious and cleaned-up Slashdot doesn't seem to be taking off. I was hopeful, but there doesn't seem to be critical mass forming up there. After an initial flurry of new story posting,
Other places like Codingstyle, which, I think, deserves more attention, seems to languish, almost completely ignored and never influential.
Orkut, which seemed to be taking off a few months ago now seems to be stalling out. There's just not much there to recommend. Those kind of sites - social networking sites - have little advantage over maintaining weblogs and keeping in touch with email, it seems to me.
I think places like Delicious - social bookmark sites - are really filling an important need and I see these becoming a really big thing. Other social bookmark sites of interest are Simpy, Spurl and Unalog (there are others, but I believe that Delicious is the "first" recent example, Clay Shirky wrote up an interesting history of such sites on the Delicious discussion mail list awhile back (search for a June 1 post by Clay Shirky).
I'm not sure what builds a community on the internet. I'd like to be able to say that if you offer something new, or offer it done well that people will be attracted and community will form around it, but I think that's too easy an answer. I think that it has something to do with appealing to egos of those who use it, making them feel special for being a part of this community, but that, too, is the obvious and probably wrong answer.
I guess if I knew the answer, especially if I'd known it before 1999, I would have bottled it and made my .com Million$.