I don't think that's actually true. There are very few nonsense posts (or at least, very few that get voted down); and when there are, downvoting doesn't always discourage the poster. When I see a post with a negative score, it's more often one that is controversial, or that disagrees with LessWrong dogma, or that was made by someone unpopular here, or that is in the middle of a flamewar between two users, or that is part of a longer conversation where one poster has triggered an "omega wolf" reaction from the rest of the pack by acting conciliatory.