What was the problem?

Since I have claimed that the derivatives involved in the financial system problems were not too complicated, what was the real problem?

There are many candidates, too many to cover right now. The use of irrelevant statistics to justify risky holdings, as I mentioned before, was a large part of the problem. The government pressure to make more and cheaper loans to less creditworthy borrowers has been widely commented on, and may have contributed significantly, but can't excuse the banks' errors.

The actual error made by the banks was very simple - embarrassingly simple, really. They bought mortgages to securitize them. They split the securities into high-risk, medium-risk, and low-risk bits. They valued the bits and found that they were more valuable than the original mortgages, which meant the process was profitable to them. They sold the high-risk bits to speculators and the medium-risk bits to long-term investors. But they kept the low-risk bits. Thats it! That's the error!

Presumably, after they valued the low-risk bits, they found that nobody actually wanted to buy them at that valuation. What they should have done was price them down until people did want them, then re-evaluate the whole business on the basis of the actual market prices that they got for them. I have no idea whether that would have meant that securitization would have carried on or not. But either way, it would not have left the financial system dangerously exposed to the housing crash. At worst, it would have ended up as the "normal" sort of Wall St scandal - clever investment bankers sell a whole load of toxic crap to investors (see auction-rate, internet IPOs, etc. etc. etc.)

Why did the banks hang onto these investments, rather than sell them? I guess that they believed they were "really worth" pretty close to par value, and that buyers didn't want to buy them at that price just because they were uninformed. Also, because they were rated as so safe, the regulators were happy to consider them non-risky for the purposes of capital requirements. That was the regulators' biggest error. Because of course these two justifications contradict each other. If the securities can't be sold at their alleged "real value", then they are tying up the banks' capital, and should be counted as such.

(I'm surprised we haven't heard more about the mezzanine tranches that were sold to fund managers, pensions, insurance, etc. They were never seen as safe, so the bodies holding them could afford to take losses on them.)

It almost seems a shame that this whole crisis is caused by one such straightforward error. It ought to be something like "derivatives are too complex" or "regulators were subverted" or "government forced banks to make bad loans". It's a let-down that it was just "banks held one particular type of investment that it was never their business to hold, just as a by-product of one business line".