Some were policies of omission, rather than policies of commission....such as leaving the shadow banking sector unregulated, continuing to incentivize home ownership. Running large deficits to fund wars & pass a massive federal prescription drug bill while simultaneously giving massive tax cuts....which went primarily to the wealthy...was also a factor.
The largest single factor was probably the collapse of the housing & mortgage markets...which were greatly fueled by those first two errors of "omission" during the Bush presidency. (The Fed under Alan Greenspan must also bear a great deal of blame by continuing to keep interest rates artificially low during Bush's re-election campaign in 2003-04 to prop up the economy until after Bush could squeak thru a close re-election campaign. To me, this was entirely politically motivated, because Greenspan had ample evidence by then that the housing markets were getting dangerously overheated....and Greenspan's delay in raising interest rates was like throwing gas on a fire.)
The American Enterprise Institute had this very good article on the subject:
http://www.aei.org/publication/gove...-up-to-the-financial-crisis-a-forensic-study/