What pique my interests are:
- Nikon designed sensor, with 45.7 megapixels and increased ISO sensitivity up to 25,600
- silent shutter modes – great for wedding photography and quiet venues
- 153-point AF system used in D5
- full-frame 4K UHD video
- tilting screen with touch capability
- 7 frames/second (up to 9 frames/second with battery grip)
And a lot more nice upgrades and features, but I find the bolded features (my emphasis) to be the most interesting in features driving the desire to take the upgrade leap. But honestly, the size and weight of the D850 camera body would too large and heavy for me. I really like the smaller and lighter form factor of my D600. Maybe Nikon would trickle down some of these features to the replacement body for D610 and/or D750, particularly the silent shutter modes and the full-frame 4K UHD video. Wishful thinking?