The velocity of collateral
From a new paper (pdf) by M. Singh:
Using hand collected data, this paper shows that, post-Lehman, two effects have occurred: overall collateral availability has declined, and the intermediation chains have become much shorter.
The paper argues that a decline in the velocity of collateral is similar in some ways to a decline in the velocity of money. Here is a related article (FT$). These are not fun or easily digestible pieces, but they are good background reading for a deeper understanding of how money markets have changed and why there is still some risk they might break down.