I liked the two ideas Paul mentioned: length of comment is related to stupidity, and not using the same words again. Just make those fall toward the bottom of the page and we'll all be happy.
While any algorithm can be gamed, length is still pretty good. If I have to go through the extra effort of adding two paragraphs each time I write something, well... it takes time to type. So I have an incentive to think and improve my comment.
Really, I like this. It's really the same problem as Google search: out of all the junk out there, how can you tell interesting content? Because we all want to read only amazing stuff and engage in discussions with brilliant people. In practice, it doesn't work that way yet, but what if technology made it possible?