A good summary from an interview with Nouriel Roubini at Amazon
The Fed kept interest rates too low for too long in the earlier part the past decade and fed — pun intended — the housing and credit bubble. Bankers and investors on Wall Street and in financial institutions were greedy, arrogant and reckless in their risk taking and build-up of leverage because they were compensated based on short term profits. As a result, they generated toxic loans – subprime mortgages and other mortgages and loans – that borrowers could not afford and then packaged these mortgages and loans into toxic securities – the entire alphabet soup of structured finance products, so-called “SIVs” like MBSs – Mortgage-Backed Securities, or CDOs – Collateralized Debt Obligations -- and even CDOs of CDOs. These were new, complex, exotic, non-transparent, non-traded, marked-to-model rather than market-to-market and mis-rated by the rating agencies. Indeed, the rating agencies were also culprits as they had massive conflicts of interest: they made most of their profits from mis-rating these new instruments and being paid handsomely by the issuers. Also, the regulators and supervisors were asleep at the wheel as the ideology in Washington for the last decade was one of laissez faire “Wild West” capitalism with little prudential regulation and supervision of banks and other financial institutions.