I think an interesting experiment would be to only allow users to submit one URL per day. Find two
really interesting URLs in one day? Submit it the next day. How many people think that they are actually going to find more than 365 worthwhile stories in one year?
This also serves the dual purpose of focusing attention on discussion, which I believe is the most important part of this site. It'd be an interesting system to try, if only to see how it turns out.
The number of new submissions correlates with the number of users. Number of users correlates with number of people looking at and voting on the "new" page.