Generally including but not limited to:
Extremely robust build quality and weather sealing
Vertical grip
Long shutter life
Long battery life
Two card slots
High end but not bleeding edge systems such as sensor, AF module, etc
100%, bright, prism finder
Lots of programmable buttons and wheels
Lack of popup flash, scene modes, and other features intended to broaden appeal.
And now we can start the litany of point-missing replies of the form 'camera X has feature Y therefore you are dumb'
The discussion is dumb. It's like you guys don't recognize that the D3/4 exist, or Canon's similar products. These things exist. They make up a product category. This product category is commonly referred to as 'pro cameras' in contrast to the 'prosumer camera' of which the D810 is a very very nice high end example.