Additional protocol enhancements are also being specified to make channel bonding less likely to cause interoperability issues. This includes things like the ability for devices to assess whether neighboring channels are clear and available for channel bonding, and for devices to reserve wider bandwidths in advance of data transmissions. This allows channel bonding to increase from 40MHz in 11n to 80 and even 160MHz under certain conditions in 11ac. Making channel bonding less invasive, and therefore used by default by wireless devices, is key to achieving the increased data rates in 11ac.
The second good thing about 11ac is that even though data rates are increasing significantly, power consumption is likely to decrease as compared to similar 11n capability, and this is a huge win.
802.11n is really starting to push the power limits, especially for mobile/portable devices, to the point where most portable devices cannot come close to taking full advantage of 11n capabilities. Through the use of more efficient data encoding mechanisms, 11ac allows devices to use fewer multiple transmissions paths while still achieving higher data rates, and it's the additional RF transmission chains that really eat up power.
Also, given that one primary use case of 11ac is video distribution within the home, power issues can be further mitigated since many of the devices will not be mobile -- like your 52-inch LCD TV -- meaning AC power is readily available. And video delivery is typically "one way," meaning a PC or DVR will be transmitting the video, requiring more power, while the TV or iPad will mostly be receiving the high bandwidth signal, which requires less power.
Lastly, 11ac introduces an optional capability where one transmitting device can send data, like streaming video, to multiple receiving devices simultaneously. Today, 802.11 traffic is essentially point-to-point. If the same video stream needs to be sent to three clients, it requires three times the bandwidth. With 11ac, the bandwidth is utilized much more efficiently.
As with many technological advances, 11ac is very complex, and this is certainly bad, especially for technologies that are targeted at the general consumer market. So far manufacturers, along with industry associations like the Wi-Fi Alliance, have done a great job in hiding complexity from end users, so there's some hope here. However, 11ac increases the number of configuration and operational options between transmitters and receivers, so clearly setting user expectations regarding the capabilities of each of their devices may be a significant challenge.