He just fails to exercise muscle in his own defense...some say character to a fault..."walk softly and carry a big stick"...only problem is sometimes you have to swing the stick.
Clinton denied everything even if it was true, Bush didn't deny anything even if it was false - that did a lot of damage.
george bush vigorously and tirelessly defended his administrations' policies in the 2004 election, and he won. he did clam up once the election geared up, and unfortunately that happened two years ago in this election cycle, but that's because he considers it improper to interject the power of the presidency into a presidential campaign that he isn't running in.
it was mccain that failed to defend the bush policies. in fact, he ran from them. he ran as obama-lite. if he had stood toe to toe with BHO and combatted the lies about the surge, the lies about the war in general, and the lies about who is really responsible for the financial crisis, he would have had a chance.
on edit : but I am NOT saying that mccain lacks balls, mind you.