Currently, you need to figure out the sorts of places where you would like to advertise. Why not provide an objective measure of visitor quality and have Google figure out which places are best to place your ad?
For example, for a social news site, visitor quality could simply be the karma of the user after a week or month.
Another measure of visitor quality would be the number of ads that the user clicks on your site. This measure could be determined solely by Google if we restrict ourselves to the Google ad system.
