"Have at least 200 karma or have been an HN member for more than 2 years" this is too restrictive.
Thus, if this is done, there could be demand for a job board where non-threshold-meeting HN members can employ threshold-meeting HN members to post their job ads. Ok, partly kidding.
Hey, I have exactly 0 StackOverflow karma (and no account, but that's easily fixed). I'm hardly surprised that they don't roll out the red carpet for me.
Perhaps a way around the restriction would be to adopt a similar system. People with 200 karma or 2 years would be automatically accepted, but others would need an invite from an existing member.