Credibility is the big difference. Everybody knew, allies and enemies both, that George W. Bush was committed to winning the war in Iraq. It's unfortunate, but hardly anybody is convinced that Barack Obama is committed to winning in Afghanistan.
Wars are contests of wills. If our efforts in Afghanistan have an increasingly ghostly quality—visible to the naked eye but incapable of achieving effects in the physical world—it has more to do with a widespread perception that we just aren't prepared to do what it takes to win than it does with the particulars of counterinsurgency strategy or its execution. Gen. Petraeus won in Iraq because George W. Bush had his back and the people of Iraq, friend as well as foe, knew it.
By contrast, the fact that we have been unable to secure the small city of Marja, much less take on the larger job of Kandahar, is because nobody—right down to the village folk whom we are so sedulously courting with good deeds and restrictive rules of engagement—believes that Barack Obama believes in his own war. The vacuum in credibility begets the vacuum in power.
You can sense Obama's disappointment. From time to time he revisits his Afghan dilemma, hoping conditions will permit him to begin his promised troop draw down, but every time he looks he finds that he can't do it. Back to domestic issues until Afghanistan intrudes upon him again, and again he engages with the only question that matters. Can the troop withdrawal begin?