The Z-Mount has a very large diameter, yet is very close to the sensor, with just a 16mm flange distance. That small gap means that bulky "retro-focus" optics aren't necessary, allowing for more compact lenses with higher performance. At the same time, the internal mount diameter of 55mm helps get more light to the sensor, especially the corners, for full illumination. That means that, despite its razor-sharp depth of field and potential for incredible bokeh, the Z-Noct will still be sharp at the edges, unlike other very bright lenses, Nikon said.
The Z-Noct is still under development and we don't yet know the price, but Nikon has called it the "flagship" Z-Mount lens -- so expect it to cost a lot. Beyond all that, however, it's a great way for Nikon to show off the potential for the Z cameras, and a good sign of fast Z-Mount lenses to come. It's slated to arrive sometime in 2019.