This is actually why I left reddit. As the site grew, I became increasingly frustrated that anything I said contrary to popular opinion was mercilessly downvoted & derided, regardless of the actual quality of the comment.
Reddit now cultivates one set of values & opinions. Anyone who disagrees is hounded & ridiculed until they leave.
I hadn't noticed this. In fact, I noticed the opposite -- that well-articulated comments taking controversial views still got upvoted. It was just a stricter quality filter. And anyway, it's easy to take advantage of reddit's predilections. A while ago, I read a great George Will column, and thought it deserved to be reddited. My headline was something like "Notorious Neocon George Will actually believes that..."
I left because it was getting boring and homogeneous.
That's why a possibility of being downvoted never stopped me from commenting, be it reddit, slashdot, or YCnews.
One lesson that I learned from reddit in particular, is that if you are interested in the topic, you should read all comments, all the way down the page, because there might be something there, in the negative karma area.
This is already how it works in open source even if there isn't an explicit karma value. If good developpers and contributors are detected they may get an offer for a payed mission on the project or even better a permanent job.
PG said for instance that a good karma could catch their attention. Some sort of benefit to getting a good karma.
It is preferable that karma benefit are not made public and taken for granted otherwise the system could be perverted.
That's why I think there are two types of winners in this game: those with good implicit ranking mechanisms (Google, Flickr) and those with good human moderation (Slashdot). Those based on explicit voting are taking the risk of being taken down by broad and unfocused masses.
Two, you could have interesting comments, based on people voting them up or down, and their replies.
Also you could do some kind of geomtric progression instead of linear, or combine that with the voter's own karma.