I think it would be interesting if users could spend their karma on performing moderator actions, perhaps with some sort of algorithmic exchange rate that converts acquired karma into modcoins.

For example, it might cost 1000 modcoins to pin an article to the top of the page for ten minutes. Or perhaps more of a bold change: 2000 modcoins to make the text of your comment glow with a golden hue to make it more noticeable. 5000 modcoins to display an image of Paul Graham at the top of the thread, smiling beatifically at all the comments below Him. And so on.

This would of course be of no interest to users such as myself who habitually generate throwaway accounts and discard them, but I would be curious to see how high karma users would use such a feature.