Intel's Dadi Perlmutter, Executive Vice President, Architecture Group with Xeon Phi and Atom CPUs
For most of the past 40 years, power consumption was treated as an afterthought at virtually every level. Unless you were building specialized hardware for particular operating environments, it made little sense to invest in clock-gating or other power conservation technology. Moore's law and Dennard scaling regularly delivered better transistor processes that leaked less and scaled more efficiently without requiring any particular effort on the engineers' part.
That trend came to a decisive halt at the 90nm process node back in 2005. Intel had already begun to develop technologies to lower CPU power consumption by that point; the company's SpeedStep technology had debuted several years earlier. Haswell continues this work, offering fine-grained control over areas of logic that were previously either on or off, up to and including specific execution units.
These optimizations are impressive in and of themselves, particularly the fact that idle CPU power is approaching tablet levels, but they're only part of the story. Operating system changes matter as well, and Intel has teamed up with Microsoft to ensure that Windows 8 takes full advantage of current and future hardware.