Sure, the new iMacs have a couple of Thunderbolt ports and more grunt between them, but other than that has much else changed? To answer that question, we'll defer to the iFixit guys, who've already torn the 21.5-inch iMac apart.
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.