Good points/suggestions all around. There are a number of solutions that strive to achieve the goal stated in the OP. Some of those we have looked at in the past were dismissed for technical reasons, while others were dismissed for "gaming" reasons.
Some might even remember our short and ill-fated use of vBulletin's built-in Reputation feature when we initially converted to vBulletin. If you don't remember, suffice to say it didn't work out quite like we might have hoped
So, while the suggestion is a fair one, we are yet to find a solution that is both technically sound and immune to those who would choose to game it.