Overheating.
4:2:0 Color Space internally.
Waveforms Vectorscope variable position focus and a more robust codec will keep the Atomos and CD Odyssey in play for serious video.
Bob
Bob, I think you nailed it.
Nobody has said anything yet as no cameras have shipped yet, but my money is down on the same internal over heating problems for anything but short clips the A7R II suffers from. The A7S II is basically the same sensor let's not forget that ran too hot in the current A7S to record 4k internal. If 5 min clips are all you want, that may not be a problem. For real production though, another option is still going to be necessary.
I can also say with certainty that comes from doing it that there is a big difference trying to capture and grade 8 bit 4:2:0 and working with 10 bit Prores 422. I had not expected the difference would be as dramatic, but the Assassin 4k files are worlds more robust and "thicker" than the 1080P that comes out of the camera. The fact that it is all a color calibrated workflow is also nice. The color is bang on, without that red channel boost the internal 1080P always seems to have. Once you start pushing those fat 4k files around there is no going back either... you get hooked quickly.
I've only had my Assassin for a week now, but after working with it the first shoot there was no way I will ever go back to just the camera rear screen unless I am out street shooing and need covert cover. Working with that big honking 7" 1080P HD monitor makes it easy for folks to spot you, but it also makes focus checking cake, and since it will use a LUT, actually seeing what your lighting, focus, and colors look like as your shooting is a real dream. My only regret is I waited this long, I should have bought the Shogun when it came out, but hard to justify the cost. The Assassin is all I need, and a great accessory for any video shooter at $1,300 in the US.
I'm crossing my fingers that Sony has been able to keep the new camera cool enough to go for half an hour at a time, and if they have I will be looking to get one. If the video isn't flawless and reliable though, I am just as likely to spend less money buying a bargain basement priced lightly used second A7S and a second Assassin for my B camera rig.