"What is the great disaster that was supposed to have befallen us..."Simply put, the thesis is that the entire world economy would have collapsed. It would have started with most of the investment banks being unable to do business and shutting their doors. This would have caused a chain reaction other parts of the financial system since the big money-center banks provide much of the overnight lending liquidity for smaller banks (and each other). Banks would start failing at an incredible rate.
As the banks started to fail, the equities, bond and commodities markets would have collapsed. All these markets rely on short term lending, not just for leverage, but to underwrite transactions. Even if this were not the case, much trading volume would have dried up. This combined would have led to a crash in those markets.
Finally, and most importantly, most businesses rely on lines of credit and other short term debt facilities to finance day to day operations. They use this to smooth out the cash flow when buying raw materials/inventory, purchasing assets and meeting payroll. These would have all been effectively frozen. You would have either seen paychecks becoming worthless or massive layoffs within a few weeks to a few months of the initial bank collapses. By massive, I mean a spike of an additional 10% to 20% unemployment within a few weeks or months.
I won't go into the effect all this would have on currencies and international trade.
All told, it would probably have plunged the entire world into a depression that would have rivaled or exceeded the Great Depression of the 1930's. It likely would have been worse because of the interconnectedness of the world's financial and commercial systems today.