Here's an idea: Only users with a certain amount of karma can submit stories.
The first few karma points have to be gained by commenting, and then for example with every 10 karma points a user can make 1 submissions. Or have different 'levels' with certain thresholds, etc. You get the idea, and I think that could help reduce the amount and increase the quality of submissions.
The obvious problem with this is that it eliminates the possibility of old-timers using throwaway accounts to submit anonymously. That's a pretty major drawback, if you ask me-- there's a lot of good discussions that begin with an anonymous "Ask HN" or "Tell HN"...
The first few karma points have to be gained by commenting, and then for example with every 10 karma points a user can make 1 submissions. Or have different 'levels' with certain thresholds, etc. You get the idea, and I think that could help reduce the amount and increase the quality of submissions.