It's an interesting question, specially because everybdy wants an answer and has an answer as well to this query. I would not put the blame on Financial innovation or American politics or Greed or Real estate crisis, etc etc... What we receive is often a result of what we choose, and we have chosen to follow a "similar to" free market economy model. Not a wrong choice but surely it comes with an inherent characteristic named *swing* which is responsible for 'almost cyclical' booms and bursts in the economy. Free market economy, though gives space to greed, profit motives, financial innovation, political adventurism, etc etc, still manages to provide an economic model which is good enough to be followed by the majority of the world. For me, the only downturn is the *swing* which needs to be controlled through appropriate policies. The swing needs to be attenuated through regulatory measures.
With the choice we have made, everything else which we have blamed so far is inevitable. Political adventurism, greed, booms, bursts, et al, have happened in past and will keep happening without control. The only controllable part is regulation which we failed to provide and thus should bear the blame for the crisis.