Would a job board decrease the quality of candidates on average? I would guess part of the reason the quality of candidates is so high is because it's active readers/participants of HN that respond/post to the threads. Having a job board would invite many other people as it is easier to access (non-active participants may miss the 'Who's Hiring' thread but a job board would be harder to miss).
There are potential solutions to this, for example only allowing users with a certain reputation point threshold to access it (I believe StackOverflow's job board employs a similar system). Just something to think about...
That's a good point, and I like your proposal, but we post on other job boards and don't currently lose much time sifting through obviously bad candidates. So, even without any such mechanism, I think that the benefits of improved interactions with HN members would outweigh the extra costs.
There are potential solutions to this, for example only allowing users with a certain reputation point threshold to access it (I believe StackOverflow's job board employs a similar system). Just something to think about...