It doesn't much matter what the barrier is. A commenting system that crashes and destroys conversations occasionally, driving away people who are not sufficiently invested. A focused remit that drives away most people who see the site. A small group that does not advertise. But I've never seen any community sustain itself in a form that I want to be part of without some barrier to limit who gets involved in that community.
I'm not entirely clear on what the reasons are. Is it that we can only track a certain number of people? Is it that communities can only sustain themselves if turnover stays low? I don't know. But I've observed the rule in multiple places.
Given that, I've been surprised at how well HN held up. It started with a good seed. People who find pg interesting have a reasonably focused remit. The site lacks a lot of silly bells and whistles. People mostly find out about it through word of mouth. But still in the end without some barrier to entry, any sense of community is doomed. At least if my experience/opinions/etc is accurate.