Sounds like a good idea. I'm thinking something along the lines of what they have for the pictures on www.pinkbike.com (atleast how it was when I last looked).
Basically, next each comment you had the choice to give the comment a thumbs up or thumbs down. The totally useless comments got a bunch of thumbs down and the good, relevant ones usually got a good rating. BUT the best part of this little system is that near the top, right before the comments section began, was a little bar that gave a bunch of options. The options were things like "View comments with +5 or more" or "View all comments" or "View all comments with +15 or more" kinda things so you could block the shit comments and only view the good ones.
That system could be tied into the karma system as well. Like if a certain persons comment got a negative rating their karma would fall slightly as a result.