Looking at it from the front, you'd be forgiven for not seeing the difference between the new iMac and the old design. If you look at it really closely, you might see that the new display has less of a glare than the old one. However, if you look at it from the side, the difference is obvious: this new iMac really is extremely thin.
The new iMac may look the same as the old one, there are some differences though. Obviously, there are differences between the various different configurations, but everything else looks remarkably similar, especially when you're seated dead straight before it. Don't try to insert a cd or dvd in the slot-in drive in the side though, because that's no longer there.
Looking at the extremely thin sides, you might wonder where all the components have been placed. If you turn the iMac around a bit more, you see that it's not that wafer-thin everywhere. The back panel bulges towards the middle in order to be able to house, among other things, the PSU.