Really anal suggestion for a nonexistent feature: square instead of circle. That maintains the compact feel, while giving the visual metaphor of putting a hold on your vote.

Somewhat related, I would really like an abstain feature not just because I like the idea of hiding votes, but because if I see an arrow I feel the need to click it until all the arrows disappear. An abstain button would help when I don't want to upvote and don't have a downvote.