Howard Fineman sees a common theme behind Obama's missteps: pride.
Another way to say it is fear. His decisions in the past few months have largely left him diminished. He's given up the speeches that won him so many converts. He gave the Democratic Party the convention it wanted, not the convention he needed. He's taken to competing against McCain's VP choice. This isn't to say he's acting pathologically, just that his missteps are those of someone unsure he wants to win, not someone so prideful he's ignoring the obvious.
Interestingly, I think McCain suffers a similar fear of success. However, he's gotten over it (for now) by picking Palin.