That's true. Maybe the issue is that when there's only a 'down' and an 'up' button, people infer everything into it. Maybe we should have an 'up' button for interesting posts, a 'flag' button for posts that don't add to the discussion, and 'agree' and 'disagree' buttons to make it clear that it's a different thing from upvoting a comment.

Then people could turn on or off some way to see which posts are heavily agreed or disagreed with without downvoting interesting posts just because you disagree. You could even have a system where you mark people you respect and then only see which posts they agreed with to get a more personal feel? Might be an interesting discussion board idea to pursue.