Man, in the low light footage, the D800 looks pathetic ISO1600+. How can it look so good at still and so bad at video?
How? Because the 5D3 samples the entire sensor for video and stills while the D800 samples the entire sensor for stills but samples only 1/3rd of the sensor for video (tosses it 2 out of every three photons it collects when it does video by only reading one out of every three lines across the sensor).
So D800 has 1.5-2 stops worse SNR at any ISO than the 5D3 (for video). And only maybe 2/3rd of a stop better ISO at 100 (vs closer to 3 for stills) and already worse DR than the 5D3 by ISO800, if not earlier, and 1.5-2 stops worse DR for video ISO1600+.
The reading 1 in 3 lines also means it has much worse color moire and a bit worse aliasing. The do grab greater than 1920x1080 video and carefully downsample to 1920x1080 and don't apply a strong AA filter in processing beforehand so it grabs a bit more details than the 5D3 (but it's still no detail champ) and it also grabs some false details too.
They need to add zebra stripigin, focus peaking and a true 1920x1080 crips 2x2 samples 1.6x crop mode to the 5D3 video if they don't want to blow it all (C300-level will be hard to compete and they won't be any sort of champions there, the 5D3-level was there claim to fame, so cripplng it to help their cinema line is really, really dumb IMO).