When historians review this period, they will have to deal with bush and obama in the same book - there's no rational basis for drawing a line between the two.
Well, there are the little problems of exploding the budget deficits far beyond Bush's worst of $400 billion, ramming through the "Patient Protection and Affordable Care" act against the wishes of the public, handing Chrysler and GM to his union cronies, handing nearly all of the $1 trillion "stimulus" to public sector unions and workers, abandoning Israel, shunning Britain, cutting off new drilling for oil, imposing a freeze on drilling in the Gulf of Mexico, 10% unemployment (5.6% was the worst under Bush, IIRC), worsening race relations, Rahm Emmanuel, Sonia Sotomayor, Elena Kagan, Van Jones, and a few other missteps.
Other than that, GW and Obama are pretty much indistinguishable.