Slashdot's moderation system is extremely flawed. It does not take into account basic human nature.
Moderators are anonymous. This leads to abuse. There is no accountability for bad moderation, and no real reward for good moderation. People will not waste their precious 5 points modding down anonymous trolls, instead choosing to mod down people with identities; people who are logged in; because they can be hurt, and their mod points actually affect someone else.
I have received a total of ONE karma for moderating in the past six months. I've scored moderator points at least 15 times, perhaps more. ONE karma, for 75 moderations done in good faith. That is my reward. The reward for those who abuse their powers, is seeing another person's karma drop like a rock. This is far more attractive to many.
Obviously, moderators should not be anonymous. Everyone should have meta moderation points, to spend wherever they please. And, they should matter, promoting the good over the bad, by giving out karma bonuses for only positive moderations.