That's an interesting point of view for the "karma economy" angle; I believe there's always enough karma sources, or rather there's always ways to create them if the scope of the site is wide enough.
I would push the idea a bit further and introduce karma destruction. To me, the most useful way to destroy karma points is to introduce a karma-based site subscription; that is, one subscribes to the site in order to read/write it but there is a constant fee of N karma points per day. User contributions are tipped with karma points taken from the upvoter/downvoter's karma pool. Eventually, one may let user buy free access (but not karma points directly otherwise it becomes a plutocratic system) in order get some support money.
They already award some karma for doing those stuff but its a very small amount and it sometimes has a cap (IIRC, you only gain karma for your firt 500 edits)
