I think the main reason /. needs the moderation system it has is because of the volume of posts. When a single topic can generate over 500 posts, it becomes impossible to read every response unless someone is paying you to do it full time. You have to have some system to help people decide what is worth reading.

The problem with their system is, as others have pointed out, that people end up modding down posts just because they disagree with what was said. When I used to read slashdot regularly, it was common for anyone who posted a comment critical of Linux or supportive of Microsoft to get modded down as a troll.  But if you were pro-open source, pro-Linux, anti-Windows, anti-Microsoft you could get away with making insulting , inflammatory posts. There was a huge double standard.  That was one of the things that made me stop reading Slashdot. Maybe things are better now. But I doubt it.

I would vote to stay away from Slashdot style moderation, unless your goal is to eliminate unpopular opinions.