But then what's the incentive for good submissions? If it's gotten to the point where essentially every article from certain sites (eg Techcrunch) gets submitted by karma gold-diggers, it might make sense to just have them auto-submitted so no one gets the karma, but they can still get discussed.

EDIT: Another thought: Only allow submissions from people above a given karma. That way you would always have to earn your stripes through commenting. Downsides: Wouldn't allow sensitive anonymous 'ask HN' posts from throwaway accounts, and we would lose all the good submissions from new accounts.