The timing of the repeal of Glass-Steagall makes this deregulatory move a convenient scapegoat for the financial crisis. But the crisis began with the housing collapse, a result of government encouragement of unsound lending practices. Financial firms took too much risk with mortgage-backed securities, in part because of moral hazard engendered by government guarantees and partly because bond rating firms were not as independent as was once thought. The limited liability that the investment banks gained when they became corporations may also have amplified moral hazard. There is no good reason to believe that Glass-Steagall, had it remained in effect, would have prevented any of these problems.
I highly recommend this piece. Lots of good history behind the law as well as a very clear explanation of the different types of banking services, what they do, and how they are created.