Dean Changed Politics
Joe Trippi claims the Dean campaign changed politics through its emphasis on the internet as a campaign tool. He may be right, but the change wrought by Dean was much greater than that, and much more ambiguous in its desirability.
His most important impact was solidifying political rhetoric as the deteminant of one's location on the left-right axis, rather than policy. Dean was a DLC Democrat - he supported the death penalty, opposed gun control, and governed as a deficit hawk. Yet he was well received on the left, rejected by the center-right, and mocked on the right, because his rhetoric, though accurate, wasn't attractive to the Heathers of the Washington Establishment.
The medium is the message, and Dean proved it.