Perhaps Obama never should have promised to "fundamentally transform the United States of America." It set the expectations too high. The political system doesn't move that fast. (In retrospect, it sounds like a promise to harness the energy of unicorns.) Recognizing the limitations such grand promises would put on governing would have represented a sophisticated understanding of his political moment. Maybe it never occurred to him that by running as a person who would be above politics he was inadvertently constricting his ability to do the job once in office? Of course, had Obama not successfully sold the idea that he was a rare figure who could unify the nation, he may never have won the election.
An alternative view is that Obama always knew that his post-partisan posture was a gambit. He knew what the politics of the office required, but by positioning himself as a transcendent figure he sought to create a political currency that he could then use in the morass of Washington.
Whichever view you take, we know that the president failed the political test he set for himself. His post-partisan age never materialized. He was not able to convince Republicans to join his health care push. He predicted it would help Democrats in his party in the 2010 elections. It did the opposite. He faced what he called a "shellacking." In the period that followed, he was weakened politically. He was unable to reach a long-term budget deal and wound up agreeing to an extension of the Bush tax cuts he had long campaigned against. He now cites this failure as the greatest disappointment of his first term.
The challenge for those who argue that Obama was naïve is to explain the obviously political moves he took. On his first big fight over the stimulus plan, Obama tried a variety of gambits, buying off Republican votes, pressuring members of his own side, and in the end going back on a variety of promises about cooperation and transparency that he had made in the campaign in order to get things done.