In the initial wave of products, devices will offer speed improvements over existing 802.11n devices by increasing channel width. 802.11ac includes both the familiar 20MHz and 40MHz channels, and adds new options for 80MHz and 160MHz channels.
As the channel width increases, the challenge of clearing the entire band for transmission becomes more difficult. Part of the challenge is technical, due to the need of radio devices to perform more computationally intensive Fourier transforms to cover the wider band, and part of the problem is that the load on the spectrum is much higher.
With many new devices supporting 5GHz channels, it becomes comparatively more difficult to identify wide swaths of spectrum for wideband transmissions. Nevertheless, a move to 80MHz channels is assured, and it offers readily accessible speeds that are faster than any 802.11n device will ever achieve.
THE FUTURE: Major Wi-Fi changes ahead
A second protocol feature used to increase speed is the use of more aggressive modulation techniques. Development of higher data rates in 802.11 has used a technique called Quadrature Amplitude Modulation (QAM). QAM works by packing multiple bits into a single time slice through the clever use of phase shifts and amplitude changes. For each transmission interval, a set of bits is plucked out from a "constellation" of symbols. To increase the bit rate, the constellation needs to get more data points.
After being pioneered by 802.11a more than 10 years ago, the size of the constellation has remained at 64 points. 802.11ac is increasing the maximum constellation size to 256 points. Moving from 64-QAM to 256-QAM provides a speed boost of 20%. However, nothing is free. With more targets in the constellation, the hardware must work significantly harder to control a parameter called Error Vector Magnitude (EVM). EVM is difficult to explain in the space constraints here but if you want to learn more see this blog post.