Whether crimes were involved in the failures of Enron, Lehman, & Bear Stearns is still being debated. All three essentially died in the same way (borrowing short and investing long, with a liquidity crisis emerging when questions about the quality of their long-term investments caused them not to be able to roll over their short term debt). Just making bad business decisions isn't illegal (or shouldn't be), but there are questions at all three whether management lied to (essentially defrauded) investors by hiding emerging problems and risks.
All that being said, MF Global strikes me as an order of magnitude worse. They had roughly the same problem - they were unable to make what can be thought of as margin calls on leveraged investments that were going bad. However, before they went bankrupt, it is pretty clear that they stole over a billion dollars of their customers' money. Now, in criticizing Wall Street, people are pretty sloppy in over-using the word "stole." But in this case it applies. Everyone agrees that customer brokerage accounts are sacrosanct. No matter what other fraud was or was not committed in these other cases, nothing remotely similar occurred in these other bankruptcies.
A few folks are talking civil actions against MF Global, but why isn't anyone up for criminal charges? Someone, probably Corzine, committed a crime far worse than anything Jeff Skilling or Ken Lay were even accused of, much less convicted. This happens time and again in the financial system. People whine that we don't have enough regulations, but the most fundamental laws we have in place already are not enforced.