Replying to Slashdot Comment Rankings

What if you could rank peoples comments in the forums. On slashdot.org you can rank comments based on their relavence, if they are interesting, informative, or funny. The higher ranked comments show up and the lower ranked comments get "hidden" so you only see them when you click on them. It also affects your karma points so you don't wan't to leave stupid comments. Overall it makes reading a thread much more interesting.