"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Running the Retina MacBook Pro at Full 2880x1800 Resolution
Those users who want even more screen real estate by tapping into the full 2880x1800 resolution mode of the display can also do so, but the option involves a workaround that is not authorized by Apple. Macworld has more details on the process, which involves using either a third-party app like the paid SwitchResX (as noted in our forums) or one of a number of free options that have sprung up such as Change Resolution.
Retina MacBook Pro running at 2880x1800
(Click for larger)
Running a 2880x1800 desktop on a screen measuring only 15.4 inches diagonally obviously results in very small text and user interface elements, but for those willing to sit close enough to their screens to make the onscreen content readable, it may be an interesting option.
As noted by developer Steven Troughton-Smith, users running Windows on the new Retina MacBook Pro can also take advantage of the full display resolution, simply setting the resolution in preferences within Windows.