If it costs karma to comment or vote, then you also have to limit the number of submissions people make. Otherwise people will spam HN with controversial topics to accumulate enough karma to vote/comment. That said, if you could only submit one link per day, and it cost karma to up-vote or comment, I think the result would be much-enhanced. Also you'd have to give new members 100 points or something, but maybe they wouldn't be able to use them for the first week.