I wonder if adding something more opinion based could help, some sort of level of quality enabled only when you vote.
Level 1 : this answers the question
Level 2 : Good answer
Level 3 : Good, intersting, informative, sexy : I like it!
Level 4 : AWESOME!!!
The ability to easily give a bit of personal opinion could encourage voting.
It could also be some stars (or mini-blender logos) to click for visual quality, understandability (this word exists?),...