The new iMac uses the Apple logo on the back for a very specific purpose. Can you guess? Hint: It's one of the few parts of the body that's plastic.
The Apple logo integrates the AirPort antenna. Metal impedes wireless signals, which is one reason the plastic white MacBook has historically had a better Wi-Fi range than the unibodies.
Beyond that point, iFixit's teardown reveals that in order to cool the desktop-sized processor, the CPU and GPU were placed on complete opposite ends of the computers and fitted with their own heats sinks. The entire system includes "six temperature sensors, three fans, and two gigantic heat sinks."
And one piece of disappointment: the iMac's mini DisplayPort is run through its logic board, meaning the computer needs to be on if you want the iMac to serve as a second monitor. [iFixit]