It might seem odd for the lack of something to count as a new feature, but in this case it definitely does. In most digital cameras, an optical low-pass filter is sandwiched against the camera's sensor. Also known as an anti-aliasing filter, the OLPF's job is to slightly blur the image coming through the camera's lens before it hits the sensor.
Although it might sound perverse, this fractional lowering of scene detail helps prevent moiré patterning in your images. Naturally though it also means that in terms of raw resolution, you're never quite seeing the potential of your camera's pixel count.
Both the Nikon D800 and D800E had OLPFs but the D800E had the effect of its filter 'canceled' for superior resolution. In the D810 Nikon has omitted the OLPF altogether which (in theory) means that it should offer extremely high resolution - at least on a par with the Sony A7R.