It is true that many economists argue that repealing 'Glass-Steagall didn't cause the financial crisis in 2008.  Here's hedge fund manager James Rickards explaining clearly, concisely, and persuasively why it did:

It was Glass-Steagall that prevented the banks from using insured depositories to underwrite private securities and dump them on their own customers. This ability along with financing provided to all the other players was what kept the bubble-machine going for so long.

Nobel prize-winning economist Paul Krugman disagrees.  He wrote in 2012:

The dominance of Rubinits in the new administration shocked many progressives, since for many the Clinton-supported repeal of the Glass-Steagall Act, advocated by Robert Rubin but opposed by Paul Volcker, symbolized the extent to which the financial crisis of 2008 was hatched in the overly friendly relationship between the Clinton administration and Wall Street. It’s true that Glass-Steagall, a Great Depression–era law that forbade the mixing of securities trading and accepting FDIC-insured deposits under the same corporate roof, wouldn’t have prevented the 2008 implosion of Wall Street. Instead, it was extraordinarily high levels of leverage at investment banks like Lehman and Merrill Lynch, as well as the holding of huge portfolios of toxic subprime mortgages by deposit-taking banks like Bank of America, that were the fuel for the conflagration. But progressives were right to feel that Wall Street had been dangerously underregulated for too long and that the entire country was now paying the price.

But Krugman ignores the fact that the "leverage at Lehman, Merrill Lynch" and the "huge portfolios of toxic subprime mortgages by deposit-taking banks" happened because the repeal of Glass-Steagall freed up banks like Bank of America to use insured deposits (or money borrowed on them) to scarf up the toxic derivatives and incentivized the investment banks to keep packaging them and selling them in vast quantities.

In any case, whether Krugman is right with respect to the 2008 crisis, his broader point remains that repealing Glass-Steagall helped create the deregulated environment which led to the casino-style capitalism practiced by investment/commercial banks throughout the early 2000s.  It was therefore a terrible mistake.