Not that this is how it must play out, but to have a POTUS that is obviously not cut from the same cloth as every single POTUS in our history carries an expectation of change. This would be true in the foreign policy arena, where a clean break from our arrogant recent past is critical to any resuscitation of our clout in the world, but also on the home front.
Though Edwards is saying more "radical" things than either of the others, he would likely run into more resistance than they would in attempting to enact real changes on our nation and our political system. Also, the ever-nauseating pundit class will prattle endlessly about how the election of Obama or Clinton is a "mandate for change"(thus giving it the OK), whereas an Edwards win would be a "victory of the base over the sensible people".