Voting apathy is a concern around the globe in every voting-based system ever created. The free market solution would be to add a sufficient incentive to it, but in this particular space it would just create a market for vote selling, wonder where we've seen that before :D
Maybe consider achievements like badges for participation in voting activities, that would create a history of participation, and Proof Of History is an essential part of the Proof of Existence as entities of the www as it is today.
With this kind of voting I think most people will be OK with something until they aren't and will see some votes that only hit one variable for all the way high or all the way low. I definitely would like to see some kind of the above mentioned items for the top 20 voting.