Slashdot's moderation system is unsurpassed, in my view. I used to be a moderator there and here's how it worked.
Most users cannot vote on posts.
Posts can reach a maximum of +5 or a minimum of -2.
Users who have reached a certain karma threshold themselves, start being allocated mod points at random. You get given a small allocation of points that replenishes from time to time.
Each mod point you are given can be used exactly once. If you moderate a discussion and then post to it, your moderations are undone but you don't get the points back. This ensures people only moderate discussions they don't want to take part in, i.e. that they don't feel strongly about.
In particular this means that voting on Slashdot is not a way for readers to express agreement.
New users receiving mod points for the first time are sent to a page that explains how moderators are expected to behave and act. It's treated as a responsibility that's earned, not something just anyone can do.
To moderate a comment you must pick a reason from the dropdown: insightful, informative, interesting, funny, troll, flamebait iirc. Each post showed its score and the last justification given. There were also "overrated" and "underrated" which I felt was a flaw in the mod system, as they altered the score but not the last justification (and the two words themselves are too vague to represent a justification). This meant you could get comments scored as +5 Troll, for example, which was pretty funny but ultimately a quirk of the design, not intended.
Moderations are themselves meta-moderated. Many more users are allowed to meta-moderate than moderate. In meta-moderation, the judgements of the moderators themselves are scored. Users are presented with a set of posts and the justification the moderator selected and asked if they agree with it. Moderators that frequently get bad scores from meta-moderation stop being allocated mod points or are given less, and as such are dropped from the pool. If meta-mods agree with your moderations, you can get given more.
You can mark users as "friend" or "enemy" which applies an adjustment to their scores. In this way users who frequently get modded down but you like their posts can be up-scored just for you.
Scores control thread collapse not just ordering. High scored replies to low scored comments are still visible by default.
Anonymous users have post scores that start at 0. Logged in users start at 1. Users with high karma start at 2. In this way users who repeatedly make good comments get rewarded with default higher visibility.
This scheme has many advantages over the typical upvote/downvote mechanism most sites use:
• Moderation is not a simple aol me-too voting system, but genuinely intended to highlight good posts.
• Moderation is not done by the site owners. It's done exclusively by the readers. This acts against groupthink as the site's readers are pretty diverse.
• The system encourages certain kinds of posts, in particular funny posts. Reading Slashdot is/was frankly more enjoyable than reading HN: the commenters were just as smart, and that smartness was often deployed towards being entertaining.
• Abusive moderators that just downvote things they disagree with were flushed out of the system.
A modernised version of Slashcode would be a good thing to have. Its approach to discussion moderation is deeply unappreciated.