One possible solution: depend more on credentials, and give people who have useful things to say / special background knowledge priority. Weigh their votes more and give them karma bonuses. Someone from a YC startup might be given more weight than someone who opines on large-scale, amorphous social problems from a generic position ("outsourcing: economic doom or natural phenomenon? Let's flame!").
I think the big thing, more than anything else, is the learning aspect. We tend to learn more from people who know a lot of stuff and have unusual experiences or abilities. Those people tend to cluster, then less smart people cluster around them, eventually leading toward decline. One way to counter that: identify those people and give them a louder voice. I don't think there's a technical way to do this that won't be gamed, unfortunately, which leads to a major scaling problem.
I mostly try to follow this rule: I mostly comment on areas related to schools / teaching / universities (I'm a grad student in English lit at the University of Arizona), books, or literature. I try to make comments that are as rooted as fact in possible; for example, when someone a while ago said there were no laptops with IPS screens anymore, I left a link to an Anandtech review of one and didn't say much else. Problem is, I don't have any good way of systematizing that or rewarding it except on an individual level.
A better solution than "giving the smarter people a louder voice" would be to "give the louder people a smarter voice."
My suggestion below is to add a good way to discreetly provide feedback. Encourage people to send messages to you about what they think of your post, with the goal of encouraging everyone to improve the quality of their posting, and also to be more thoughtful about up/down voting.