I never felt any barrier to posting on HN. Nearly everyone on this site is pretty welcoming. Just comment on something that interests you, or that you know something about. By the way, for me, and I suspect many others, it's not really a free rider problem like you describe. I don't gain much by simply having a collection of interesting links, nor do I feel like I'm doing a service when I post a comment or article. I…
But the point is that the vast majority of HN users are likely not contributors. Instead, they're passive visitors to the site. With respect to that group, unless you don't want those users contributing, there's definitely a free rider problem.
Although I certainly don't think the barrier is unique to HN (although downvoting adds some pressure to come with something decent). I was just wondering what creative strategies people have used to get people to leave their first post or comment. Because I think once you get that first comment, you get the user hooked (b/c then they experience the added value of the discussion you mention).