I think limiting to HN users is good. Given HN is a free community, it might be nice to see some HN information about the user (who they are, how long they've been a member, links to posts/comments, karma, etc).
If I'm crunched for time, I'd probably be more willing to help somebody who is making HN a great community in favor of helping somebody who joined 35 minutes ago to get free consulting.
(Note: Unless abuse occurs in the future, I would prefer to choose myself rather than having a policy that says "You must have XX karma or be a member for N months.")
I think it would be good to also limit by karma score too. It's trivial for someone to come here and create an account. I'm not sure what the cutoff should be, but probably low enough so as not to exclude people unnecessarily, but high enough that it weeds out trolls and people who are just not that serious.
I'm not so sure about that... If you look at my profile, I almost never comment or link anything. And yet, I have been a dedicated reader since I first found HN. I have also done as much as I can to bring new, intelligent users to HN. But none of this is reflected in my karma score.
Though I could be wrong... Maybe I should just start contributing more.
