You're right that the people making decisions need to have their intrests aligned with the success of the DAC. Wealth is a metric to use but perhaps it's not the be all and end all. I think the following stats could be useful as part of an equation to determin how much 'skin' a member has in the game.

Members activity score
Give members an involvment/activity score calculated by how active they are with voting, proposals, giving suggestions ect ect

Members peer rating
Allow members to endorse each other in some way.

Thats all ive got for now, ill keep thinking onit.