Is there any incentive to hoard karma? Maybe we need to keep two values: Earned Karma and Karma Balance. In that case the only cost of sacrificing karma is opportunity cost, but I guess there will be only so many projects.
A more interecting question about up/down-votes is whether a person can change one's mind after using karma/voucher in voting. On the one hand, LW should allow people to change their mind; on the other hand, it may create some strange dynamics if someone wants to support more projects than they can with the current amount of karma.
Currently, LWers get +1 karma for a comment upvote, and +10 karma for a main post upvote. But clearly, there are other valuable things LWers could do for the community besides writing comments and posts. Writing isn't everyone's forte. Why not award karma for doing productive non-writing things? It's probably not optimal that karma and the community status that comes with it are awarded only for the thing that myself and a few other people are good at. For example, I really wish LW could award karma to programmers for improving LW.
The challenge is doing it fairly, in a way that doesn't alienate too many people. But there might be a workable way to do this, so let's explore.
Perhaps tasks could be assigned karma award amounts by LW editors (Nesov, Eliezer, Louie, etc.), or even just one person who is appointed as the Karma Genie.
Examples: