Give them more advice and guidance on the submissions page. http://news.ycombinator.com/comments?id=20487

Drop karma. Still have voting on threads and comments but don't collect that up into a public karma for an individual. Karma is clearly a motivator for some compared to peer respect from posting a well thought out comment. If ones comment goes negative it's more likely to be upsetting if you think it has hurt your karma, whereas without karma you'd just take it as feedback that people, or even a clique, don't agree. You'd be less emotive in replying.