Perhaps the most important thing about upvotes and downvotes is how they affect visibility. Everyone wants their voice to be heard, and some people want the opportunity to influence whether other people's voices are heard or not, e.g. by flagging stories or killing comments through downvoting.
If the big deal here is visibility, then I would concentrate on the algorithms that decide when a comment thread is rendered gray or invisible and the algorithms that decide the ranking of comment threads. I would look for patterns of votes or commenting that might help distinguish "popular but fluffy" from "popular and thought-provoking."
I can't help but think in the case of RiderofGiraffes particular case karma might have kept him in the community longer than he should have, or burnt him out. Getting to the point where your contribution to a site is the auto-posting of popular content is probably a failure of the system.
In general karma serves as an incentive to contribute, but its a fairly shallow kind of contribution, and I don't this site needs that anymore. Hopefully comment and submission score without accumulation give enough encouragement to quality without encouraging quantity.