Suggestion: A certain amount of karma is needed to submit links! It's easy enough to get karma through comments. That would probally help a lot on the signal-to-noise ratio.
I think it would be great if user registration happened in seasons. I.e. every three months, there's a 48-hour period during which new people can sign up. The purpose of this would be to allow the site to retain a unique personality. If you constantly add new people, it's hard to know who's who and what we're doing here. Treat it more like a large and semi-exclusive collective, and give it a chance to center itself for a while before expanding again.