By presenting a barrier to entry for accessing the content, the volume of discussion is reduced since a smaller portion of the community can get to it. This is particularly undesirable when a free alternative may exist.

The quality of the discussion may also reduce, as the non-subscribers can only respond to commenters without benefit of context of the original material.

Should enough of these posts exist, the effects could impact the community at-large; resulting in a decrease in diversity of opinion, and the popularity and relevance of HN as a site. This, in turn, could give rise to, or cause a traffic shift towards, more open competitors.

from OP:

> I frustrate when I clicked to a title or entry and when I go to the tab I see that I need to pay for a subscription to see the content. I think this creates a great struggle in an open community like HN.