Replying to American economy question

Maybe one of you can help me,

I know some one to the current economic crisis in the US was the sub-prime mortgages and bad lending, but what are some others. Would a high public debt be one? or overspending? and if so how do they hurt the economy?

I'm writing an essay that is essentially arguing that the American economy will continue to suffer unless they can implement policies that will stop business or the government for that matter from making unsound economic decision. I have one point on the mortgages or lending but any suggestions?