I've had a few days to play around with the D850 and I've come to some conclusions about it. Comparing 24mp to 46mp is like comparing a sound recording that has been sampled at 44Khz to one that has been sampled at 96Khz. It depends on the microphones, the studio, the mic pre-amps and most of all the quality placement of the microphones. If everything in the audio chain is up to the quality of the 96Khz recorder then you should hear a pretty dramatic difference. If you are using a D850 with a 24-120 kit lens, you may not see much difference between 24 and 46 mp but if you are using a high quality zoom or prime and with the right subject matter you may see a difference. Do I need 45 mp? No but it's nice to have if and when I do.
The D850 has so many nice features and is extremely customizable. One setting I like is to set the Raw size to crop and enable the mask. This give me a 21mp crop frame with the advanced focusing system of the D5-D500 that will keep snapping at 7 fps for as long as you hold down the shutter button. This should work pretty well with my 150-600 and wildlife. I can switch to 46mp Full Frame by holding down the record button and one click of the control. Custom Settings Bank/Photo shooting menu bank has a fairly steep learning curve (at least it does for me) but it is really awesome once you get the hang of it. Like others wrote, it will point out any flaws in your glass and techniques. My old Tokina 28-70 f/2.8 looks great in the center but it falls pretty short on the edges. And the raw/dng files have a lot of CA but the jpegs are clean. The in-camera JPEG correction is outstanding.
It is a bit flakey about aftermarket lenses and grips. All of the controls on my Vello grip accept for the Shutter Release and Focus button stopped working last night. I was ready to send the grip back when I tried doing a reset on the camera. After that it started working just fine. My old Tokina 28-70 f/2.8 was focusing just fine this morning but decided to stop focusing this afternoon. I mounted a Nikon AF-S lens which focused just fine and then the Tokina started focusing just fine. FLAKEY