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.
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.