As mentioned in many other comments - there is no good way for a commenting/voting system to determine a user's motivations for voting (in either direction).

...But there are subtle ways to provide incentives for user voting behavior.

For instance, why not attach greater risk/reward to voting?

Incomplete ideas that come to mind: 1)scarcity - finite votes force greater selectivity in voting 2)vote transparency - anonymity permits nastier activity 3)user transparency - highlight ornery vs constructive users

As for doing away with downvotes: Zero is an artificial floor that would simply bundle mediocre comments with comments that are off-topic/spam/in poor taste etc.