Well when you boil it down, derivatives are financial instruments or contracts derived from actual assets (ideally).
The problems come from any number of directions, a lot of it with the OTC / black box situation, where the derivatives may or may not be actually backed by something other than insane amounts of leverage, and with the same assets being packaged and sold in different ways to multiple investors at the same time.
The big scare is that if there is too much action towards recouping the investments made in derivatives at the same time, the financial system will collapse. We'd probably see a financial system freeze by the larger western governments in that case to prevent all of the value from going to zero, but yeah, with what functionally amounts to derivatives backing derivatives backing derivatives, there is enough vapor wealth out there to equal GLOBAL GDP for many decades. 85%+ of current derivatives value has accrued in the past 20 or so years IIRC. Did the global economy suddenly multiply in value by several orders of magnitude in two decades? Nah, it's just people playing with numbers.
Many volumes could be written about the issue, but they've absolutely exploded over the past two decades.
http://www.forbes.com/2003/05/09/cx_aw_0509derivatives.html