It's a nice idea, but like others have said cheating would be rampant and difficult (if not impossible in some cases) to curtail. I think the best they could hope for was something that sent sites getting too high a percentage good or bad clicks to a human at google who checked to see what the site was about and gave out bans or some bonus for the site, like a link in a "Google's Top Sites of the Month" or some such.
Doubt they'll ever get around to that though, manpower costs money.