It’s one of the most idealistic and ambitious TV operating systems ever.
But that idealism leads to strange decisions. Apple is very proud of the fact that the entire tvOS interface has been reworked to run in 4K HDR, but it also won’t let the device switch modes on your TV: from 4K to 1080p, from HDR to SDR, or from a 24Hz refresh rate (for movies) to a 60Hz rate (for games and interface animations). That means that, out of the box, any non-4K content you watch will be upscaled and processed into 4K HDR, running at the highest refresh rate your TV supports, which is usually 60Hz.
The Apple TV also automatically preferences refresh rate over any other setting: if your TV supports 60Hz HDR10 but only 30Hz Dolby Vision (like 2016 LG OLEDs), the Apple TV will pick HDR10, even though HDR10 looks worse than Dolby Vision. Apple told me that’s because it wants the interface and games to run as smoothly as possible; it’s found that the interface judders at 30Hz. So you’ll get worse HDR but a smoother interface, all because the Apple TV won’t switch modes.
All of this adds up to a real devil’s bargain that wouldn’t exist if the Apple TV would simply switch modes on your TV. I asked about it, and Apple told me it thinks mode switching is “inelegant,” because TVs often flicker and display built-in interface elements when they do it. (There’s some classic Apple-ness here.)