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.