Remember when I suggested here http://forums.linuxmint.com/viewtopic.php?f=29&t=151147 :
10- More Cinnamon effects like desktop cube and transparent window
Implemented here http://cinnamon-spices.linuxmint.com/extensions/view/25
"This extension makes the current window transleucent whenever it is dragged or resized, adding a nice bit of eye-candy to the desktop. It can be installed via the Cinnamon Extension Manager. Currently, there are no settings options to change the opacity level, the duration of the transition, or the transition delay. However this is readily accessible in the extension.js file and such settings may be incorporated in the future."
And it works out of the box and I think it should be incorporated in the Cinnamon effects