I would use the same form factor, but give each story a unique decay rate based on word count. You could also take the ratio of pageviews to upvotes into account. That problem with HN is that by the time enough people have time to read a longer story it's already gone forever, so all we really get on the front page is the text equivalent of lolcat pictures. By keeping longer stories around for longer on the new page (and then on the front page) you are putting everything on more equal footing.