It might be interesting for you to look at the rating system that's used on the site RSDN.ru (Russian Software Developer Network). The site has been around for quite some time and they've formulated a very balanced approach to rating.
On that site one has 7 buttons to rate a message: 1, 2, 3, '+', '-', '+1' and ':)'
1, 2 and 3 affect score (=karma) of the message and its author based on the level of the person who rates it (i.e. 2 from a 5th level person would add 5*2 = 10 etc)
'+' and '-' show that you agree/disagree with that comment, but do not affect the score.
'+1' adds 1 point to score independent of rater's level.
':)' shows that you find this comment funny.
I translated rules from this page
http://rsdn.ru/forum/Info.aspx?name=info.forum.rating
and as you may see, there are more rules that help to prevent excessive you-rate-me-I-rate-you behaviour, keep score up-to-date (your level depends only slightly on your all-times score but greatly depends on the marks that you've got during the last month) etc.
If people are interested, I can translate the rest of the rules.
This has some of the same qualities as Slashdot, where you tag one of several reasons on your moderation.
EDIT: I just thought of a more user-directed mod/metamod system along the lines of Slashdot, but this way would be more Web 2.0. (As opposed to web 1.99a like Slashdot.)
Have a choice of pre-defined tags (about 10), each with a karma score attached to them. These tags would be long the lines of: "Funny" "Interesting" "Insightful" "Logical Fallacy" "Troll" Users could choose to either moderate a comment, which would be attaching 1 or 2 tags to a comment and thus altering the comment's visibility rating either-or meta-moderating, which would be voting on the fairness of moderation. So you could see what tags are attached to a comment and vote fair/unfair on each of them.
Now here's the rub: you get Karma by fairly moderating. (And lose it by unfairly moderating.) So while moderating changes a comment's visibility, only meta-moderating affects Karma.
Karma would be rewarded by the number of tags one could apply to a comment.
EDIT: More ideas. Users should be able to define their own tags, but only the admins would decide what point score should be attached to them!
Some of these ideas are are instances of the pattern: redirect more of what would otherwise be a cluttering one-word 'attaboy!' or 'haha!' or 'this doesn't belong' response into a single-click and running-tally.
I see that RSDN.ru has done that with ':)'; Facebook has done it with 'Like'; HN and others have done it with 'flag' (though this flagged tally is usually non-public).
On that site one has 7 buttons to rate a message: 1, 2, 3, '+', '-', '+1' and ':)'
1, 2 and 3 affect score (=karma) of the message and its author based on the level of the person who rates it (i.e. 2 from a 5th level person would add 5*2 = 10 etc)
'+' and '-' show that you agree/disagree with that comment, but do not affect the score.
'+1' adds 1 point to score independent of rater's level.
':)' shows that you find this comment funny.
I translated rules from this page http://rsdn.ru/forum/Info.aspx?name=info.forum.rating and as you may see, there are more rules that help to prevent excessive you-rate-me-I-rate-you behaviour, keep score up-to-date (your level depends only slightly on your all-times score but greatly depends on the marks that you've got during the last month) etc.
If people are interested, I can translate the rest of the rules.