The single best highly scientific explanation as to why polys go dead, trampoline, lose control!

http://twu.tennis-warehouse.com/learning_center/deadstrings.php

The article is rather long and highly detailed. Alot of scientific experimentation here. In summary, the high "perpendicular" stiffness of polys create the effect of a higher deflection angle and increased dwell time.

Also, the crosses slides more in a poly setup, rather than notch as they do in multis and nylon setups. This creating more sliding friction which prevents the string from snapping back. So there is a perceived loss of power and mushiness.