0 - Ability to comment on threads
50 - Ability to upvote comments
500 - Ability to downvote comments
1000 - Ability to submit articles/stories
2000 - Ability to downvote articles/stories
etc. While this may reduce the number of incoming stories, perhaps there could be a way for power users to sponsor stories submitted by those who aren't able to submit them to the feed themselves. The more I think about it, the more I like this approach - create a queue of "pending stories" that anyone can submit to, but only those who have sufficient experience on the site can approve them (or remove them from the queue).
For those who say that I'd be pandering to myself here, note that I'm at 620 points right now - with this proposition I'd be reducing my current abilities. However I think that it's a small price to pay to improve the quality of submissions.
By requiring some karma to upvote people would probably contribute more low quality comments hoping to gain the karma required to contribute in other ways.