this was something I loved about slashdot moderation. When voting, people had to specify the reason for the vote. +1 funny, +1 insightful, +1 informative, -1 troll, -1 misleading, etc.

That way you can, for example, set in your user preferences to ignore positive votes for comedy, and put extra value on informative votes.

Then, to keep people from spamming up/down votes and to encourage them to think about their choices, they only gave out a limited number of moderation points to readers. So you'd have to choose which comments to spend your 5 points on.

Then finally, they had 'meta moderation' where you'd be shown a comment, and asked "would a vote of insightful be appropriate for this comment" to catch people who down-voted out of disagreement or personal vandetta. Any users who regularly mis-voted would stop receiving the ability to vote.

I don't think this is directly applicable to a federated system, but I do think it's one of the best-thought-out voting systems ever created for a discussion board.

edit: a couple other points i liked about it:

Comments were capped at (iirc) +5 and -1. Further votes wouldn't change the comment's score.

User karma wasn't shown. The user page would just say Karma: good. Or Excellent, or poor, or some other vague term.