The problem is simply one of scale: communities don't scale up easily past a certain point.

Here's a heretical idea: you're only allowed a fixed number of posts per day, and you have to increase that limit by being a member of the site for a period of time, and be in good standing. It isn't a great idea, but I think we need to strike down the Web 2.0 notion that everyone should be allowed to talk at the same time. pg's realized (thankfully) that 'heavy-handed' moderation is necessary - no community can ever police itself, it will always degrade into memes and groupthink.