Make it possible to lose karma by submitting garbage stories, either via downmods or (IMHO the better option) by making submitting a story "cost" a certain number of points of karma (which of course will be regained if the story gets voted up).
Recently I've seen two trends, both of which significantly diminish the value of Hacker News:
1. Some users are flooding Hacker News with submissions (in one case I counted 18 submissions in one day), and even though most of their submissions aren't being voted up, enough submissions are to make them accumulate lots of karma (which I assume is why this is happening).
2. The same stories are being posted many times by different users. I'm sure this is partly the result of #1 -- with the floods of submissions users might not realize that a story was submitted before -- but the fact that there's no "penalty" for useless submissions probably contributes as well.
I think there's a unique URL filter on submissions; but not a unique story filter on submissions (which would be a rather difficult AI problem).
Even with different URLs, there's really no need for 10 different stories about the MacBook Air to be posted here -- it would be much better to have one Hacker News item and have URLs to other articles posted in comments.