Wall Street was the driver in buying up these shitty mortgages to put into MBS/CDOs -- and quite a few big banks bought mortage originators so they could get bigger margin on these deals. Unfortunately for them Wall Street couldn't find buyers for all of this toxic crap and/or were too dumb to realize how bad some of this stuff was (ie Citigroup). That's why you had a bank run on Bear Stearns, once people realized the collateral they were using in the repo market was effectively worthless.
http://www.washingtonpost.com/wp-dyn/content/article/2008/08...
Without Fannie and Freddie and the backing by the US taxpayer Wall Street would not have taken as much risk.
What mattered was the implicit guarantee provide by the government through Fannie & Freddy's involvement.
You might have notice that became explicit guarantee at the end.
* No money down loans
* No proof of jobs required
* Fannie & Freddie bought this junk
* Repeal of Glass Steagal
All the talk in the world won't change the fact that the solution requires legislators and law enforcers to have a strong backbone and prosecute those who committed fraud.