The promise of Wall Street is that you can make decisions and investments and if you are right you are free to make as much money as possible. The ugly flip side is you can also lose big when you are wrong. That is what happened here, MF Global took trading positions based on expectations of future events and they were wrong; consequently 2800 people will be unemployed, bond and stock holders will lose their investment, creditors such as banks will probably receive some money back but not 100 cents on the dollar and life will go on. The good parts of this event are that the system is not threatened because MF Global is not a bank, customer monies seem to be intact, investors everywhere will question their investments and counter parties more closely and Wall Street firms will examine their own risk assessment procedures to make sure they have control over their exposure to the markets.
The broader lesson is we should make sure the system is protected by separating the banks and brokers. Capitalism is a harsh system because failure is swift and brutal and people get hurt both by losing jobs and losing money. We cannot have it only one way; if you want to succeed you need to be allowed to fail no matter how painful. Welcome to the big leagues.