I understand you can choose an answer with the checkmark, but occasionally you come across an answer that you feel deserves a heck of a lot more than that.
Maybe users can get expendable "super-upvotes". When used, these would give the target user a ton of reputation. They could be earned by obtaining enough reputation points (perhaps 1 for every X reputation you earned?)
It would provide users with more incentive to participate, as they would be getting these tangible "items" that could be used with great effect. And users who are rewarded with them would feel pretty awesome about themselves.
And for even more fun, on the darker side, a "super-downvote"...