We all have different needs based on what/how we shoot. For me the 850 may finally be a single body to suit all my needs. Which really simplifies traveling to and shooting in remote locations. In FF mode it will shoot 7fps which is sufficient for most wildlife(not too long ago I was thrilled to death with the D300 at that fps). If it's consistent with previous 800 series it will probably shoot a frame or two faster in crop modes(1.2x or 1.5x).
Same MP count as some medium format bodies. And shooting in 1.5x crop mode(i.e.DX mode) you're still shooting equivalent pixel count as the D500 with the same focus module, likely better ISO performance, etc. With the built in crop modes shooting with the 200-500mm lens effectively gives you 200-750mm EFL. That's pretty cool.1. High megapixel count
Depends on what you shoot. For wildlife/sports better ISO/noise performance always helps.2. Extended ISO range-I've only seen one camera (D500) that benefits from this range.
And true slowmo. I've never shot video with any DSLR in part because without slowmo I don't really care.3. 4K video
Not sure that's actually a benefit. I wish Nikon would just pick one.4. Multiple media formats (1XQD, XQD, SD, SDHC)
That comparison must be for DX bodies. It's only 35 grams(just over an ounce) heavier than the D810.5. 915gm body-about 200gms heavier than previous versions.
That said, I'm pretty content with the performance of what I'm currently shooting. The single biggest driver for me would be the simplicity of a single body for all purposes. Will have to wait to see what sensor performance looks like to see if that is indeed a viable option. Then decide whether it's worth the net cost of upgrading.