In busy forums, moderation can be a useful tool for gently enforcing good behavior. Drupal is pretty flexible in its support for moderation, but it could go farther to make moderation more effective.

Plastic.com, to take one example, uses moderation in ways that are different from Drupal's in some major and minor ways. The two most important differences are: 1. Limited pool of mod points, and 2. Karma points accumulate.
---
1. On Plastic, there is a limited pool of mod points--that is, you can only moderate others when you have mod points to expend. These are handed out randomly, in batches of 12, to users with a certain karma level (10 pt, I think). Unused karma points expire after 4 days, so users cannot horde them. This discourages users from moderating frivolously--they take their mod points as a responsibility.

There is not a 1:1 correlation between mod points and karma points: downmodding costs 2 mod points; upmodding costs 1 (all moderation on Plastic changes the recipient's karma in half-point steps, which strikes me as needlessly fussy).

2. When a user is upmodded or downmodded, those ratings are added into a cumulative karma rating that includes karma accrued (or lost) through forum comments, submitting news stories that make it through the submission queue (different kinds of stories are scored differently, to encourage more traffic in underpopulated forums), or even at the whim of an admin, say, if the user makes a helpful comment to a story in the subQ.

Karma points are always visible: the points assigned to a comment are always shown by the comment title; a user's cumulative karma points are shown in the user profile.

People like the approval of their peers, and this visibility of karma is a factor in encouraging forum civility (IMHO).

Plastic's system, much as I like it, could be taken farther. In addition to showing accumulated karma, it could show indices of average karma and volatility. A high karma total isn't necessarily evidence of greatness--it might only be evidence of frequent postings with occasional hits. An average karma/posts would be more useful.

Likewise, there are some people who are very controversial--they attract a lot of upmodding *and* downmodding. For these people, it might be helpful to admins to have a volatility index, which could perhaps be calculated this way:

volatility = (positive points earned + negative points earned)/positive points