In addition to a 17 percent smaller profile (fewer materials), it sports a 135 power supply, whereas its predecessors were powered by power supplies that were rated from 203 watts to 150 watts (there have been a couple of revisions over the years). Also, its standby power draw has been halved, from two watts to a mere one watt. Hey, every little bit counts.
Catch all the changes in this post over at Joystiq.
Source: Joystiq (with its swank new look, no less) Image credit: Microsoft