Not just aimed at still photographers, the D850 wants to be a multi-media production tool for videographers and filmmakers. It can shoot up to 4K UHD @ 30p or 24p and uses the full sensor width (there's no crop factor to consider with your lenses). If you use the HDMI output rather than internal memory cards for recording, you can capture uncompressed 4:2:2 8-bit 4K UHD footage, which is both great to have, but it would have been better to hit 10-bit to match the HDR capabilities of modern 4K displays (like the Panasonic GH-5 and the newly announced Sony A7R III).
Though recording internally results in a more consumer-friendly bitrate, I quite liked the Nikon D850's 4K video quality. Colors are accurate, and low-light performance is better than most cameras. There's no S-Log, of course, but you can use the camera's Picture Modes to shoot your video flat and neutral, leaving it ready to be color graded after the fact. The 5D Mark IV has a better AutoFocus system and higher bitrate files, but the motion jpeg codec is less than ideal.
The real bummer with this camera's video performance is that Live View AF system, which is accurate, but slow and, depending on the lens, way too loud. Honestly, you can't touch-focus while recording or let the camera track subjects because it'll hunt and miss and you'll hear the whole mess. But with locked focus, for interviews or B-roll, the actual final product will outperform just about every 2017 camera save for the Sony A9, Sony A7R III, and Panasonic GH5.
If Nikon could every implement an honest-to-God Live View AF system, like Canon's Dual Pixel or Sony's phase detection system, they could make one of the best all-in-one cameras out there. Yet, for a stills-focused DSLR, the 4K videos files are manageable and attractive. A flip-out or 180-degree tilting viewfinder would also help for interviews and vlogging.
In Full HD 1080p, the D850 can record up to 60p for native files as well as 120fps for creating in-camera slow motion (the 120fps shots are saved as 24p files for playback). There's also a built-in stereo microphone, inputs for headphones and an external mic, as well as a new audio attenuator to regulate sound levels.
Overall, the in-camera slow motion videos are a LOT of fun, but at a full 120fps, the action can stutter a bit and feel less fluid at times. The new GoPro HERO 6 has smaller, noiser image sensor, but offers much smoother slow motion.
The in-camera mic is about as good as DSLR mics ever are (aka NOT good), and the audio attenuator liked to record a bit hot with an external mic, but being able to set the levels is nice. Then again, it's a feature buried in the Video sub-menu.