One lesson should be clear, and the Bush administration should have listened to the hard right:
Do not let poor people buy homes. By lowering the bar for qualifying for home ownership, you'll get a certain percentage of "riff-raff" who will never be able to manage home ownership.
Keeping the down payments high, keeps the irresponsible out. This also keeps the poor out.
You always take more risk when you give poor people loans.
The Bush administration and the Republican congress are guilty of trying to help increase home ownership in the lower economic rungs by lowering the bar for loans.
This is why, I might add, that only economists are bitching. Most of the politicians are fairly quiet. It was a noble experiment that failed.