Megan has a very insightful post on the limits of Obama really sticking it to the Republicans over their record. Bruce Bartlett and I are free to do so with gusto because we both attacked Bush for over-spending and borrowing from the get-go, and opposed the Medicare Prescription Drug budget-buster (but Obama didn't do any of the above). Yes, the Bush tax cuts are hard for Obama to bash because he has continued them, as Megan notes. But he has continued them only because ending them in the middle of a severe recession would have been nuts. But bashing tax cuts is not politically very potent at any time and would open Obama up to the usual tirades from the right.
Obama should be able to blame the Iraq war for a trillion or so (I can't; I supported it) but he's commander-in-chief now and cannot easily decry a war which he is now committed to conducting and ending. Hence his political predicament.