I think a new user should, by default, get "read-only" access. Once the account ages, so the user sees what is acceptable behavior, should you get write-access.
Another idea is to actually make good the name of the site (Hacker news). EITHER a) Show us you actually are a hacker-- do you build things, or just troll? Is your relation to technology deeper than "I read techcrunch?" This could be a simple matter of adding a text-field or a mandatory homepage/startup URL field, and asking (say) 3 longtime HNers to decide if the "applicant" is interesting enough to the HN community.
OR b) get invited by a long-time HN-er to join (There should be a strong disincentive to invite indiscriminately: for instance, everytime a person you invite gets downvoted, you lose 0.2 karma points).
I know, this scheme sounds elitist. And it is. Yet, I can't think of a single interesting HN-er this would filter out.