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.