I have a 2014 15" retina MBP that I've hooked up to my old 2009 27" iMac using it iMac as a monitor via Target Display Mode while the MBP is in clamshell mode.
My problem is that the iMac turns really hot very fast. A lot hotter than it turned when I was using the iMac as my primary machine.
The iMac is running a fresh install of OS X Mavericks so there shouldn't be an issue with a lot of background apps running.
Any ideas on why this could be?