What really defined the crisis was its systemic aspect of a large bank failing and pulling others with it into the abyss, also known as the too big to fail-problem. If a too-big-to-fail bank runs into difficulties, the government will have to step in to prevent a systemic crisis.
This entails an unhealthy asymmetry to the detriment of the taxpayer – heads: banks win, tails: taxpayers lose. This asymmetry in turn provides distorted incentives for banks. Knowing that the government will have to save them, banks have incentives to engage in risky activities. At the same time, the implicit guarantee by the government makes such banks look less risky to investors.