I really like the idea of having a pledge bank and keeping people who are making regular, albeit small, donations informed about what WMF is up to. If we can say, "Your donations allowed us to run a number of workshops in South Africa. That was 3 months ago, the Africaans Wikipedia has grown by x hundred articles since then." My objective here? You're showing donors that their money is making a difference, even if - with English WP at 2 mill+ articles - they never notice the difference.
The problem is, most of the money goes on just keeping the servers running, it doesn't go on anything interesting.