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.
This approach may work if there is additional points given by the system to the upvoter (or downvoter) if there is significant number of other users doing the same thing as that upvoter (or downvoter) too, which means his/her action is indeed valid and objective.
Users would then be more willing to vote, with the hope that other users will do the same thing as him/her too, and reap more points than what he/she spent for upvoting/downvoting. This enforces more thinking and evaluation of comments or stories before even upvoting or downvoting them.
Submissions should hardly give any karma. That should take care of most of the problems regarding submissions.
About the approval: It should probably be the other way around. Flagged submissions are added to a queue for review (maybe letting the community to vote once more and therefor reviving the submission) and duplicates with comments are merged.
Random mumble: I have the feeling that many discussions depend too much on the poster. Sometimes complete discussions shift because of a comment of an established poster or live and die with said poster. And it's from time to time not because of some valuable insight but rather the fact that it's him/her. I am probably the only one who feels like that and I don't have a solution (if it needs one). Hiding the username and having to do an extra click (profile e.g.) to see who's behind the post would probably minimize the problem. But it would make it harder to filter out posts (which would need another indicator karma/per comment ratio (e.g.) to make it at least a bit useful).