Since average Slashdot posters will moderate all the comments, it seems that whatever sounds somewhat informative to an ignorant reader will quickly get moderated up even if the information was entirely bogus. If this happens, it's useless to moderate down because it'll just get moderated up again. What seems to work, at least to some degree, is to post a reply telling exactly why the comment is bogus. If the reply sounds somewhat believable, the parent tends to get moderated down rather quickly.
The most interesting thing I've noted in the moderation system is the vast amount of anger and hatred it creates. When one reads at -1 treshold, it's full of angry posts complaining about unfair moderation. Some are obviously cheap trolls but there are also plenty of people with serious content in their comments. Now, if you write a comment with serious intent and put some of your own effort and insight to a comment, and then you get moderated to -1, it's annoying. What's even more annoying is that you have no idea why it was moderated down nor can you know who moderated it. The only thing you have is a few words like "troll" or "overrated" and that's it. It's almost as if a stranger came to your door and said "Hi. You suck. HTH. HAND." and then left your house as if nothing had happened.