I think the only real solution is to have a small community whose members are actually heavily invested in the topic... basically, experts.
If I try to go add to a discussion about some new Myspace feature, my thoughts are likely going to be off topic, misguided, or an outright waste of space... all because I don't have a Myspace account, and never have. For the same reason, if a bunch of Joe Sixpacks show up on Hacker News to discuss startups, they are probably going to bury any thoughts from actual startup founders or people involved in the 'scene' (whatever the scene is).
I'm not sure how you maintain that, short of making a community invite only. Even Hacker News is starting to show the pain of size. The front page often has pretty off topic stuff on it, when I, for one, came here originally to keep up on the startups funded by YC and the thoughts of the people involved in them.
Sure, PG still comments here regularly, but how long until this place grows and he becomes an absentee owner like CmdrTaco? He may not stop commenting, but there will be so much crap in the comments that few may notice what he says.
I share the same point of view, and that's what I'm building right now with my new startup. There has to be a balance somewhere. Too small, and you get a dead community that rarely updates. Too big and it loses focus. The sweet spot is somewhere in between.
Commitment to a community is completely independent of investment in a topic.
That being said, I think algorithms can be very effective as intelligence amplifiers, allowing us to make higher level decisions and micro manage less.
Although I will admit that moderated solutions almost always come out better. See slashdot's complete lack of this problem.