Created an account because of this issue, so hello all.
In regards to how noticeable it is, ie. how big of a problem it is, yes, this will obviously vary a great deal from person to person. As someone said before, whether you enjoy this sort of adaptive contrast / brightness is largely a matter of personal taste, though I can say with some certainty that most videophiles would rather their players add as little molestation as possible to the final image (short of good upscaling, obviously). The blownout homescreen icons are the most jarring example, because we know exactly what these should look like. That much adjustment was just being made to the video, but it's much subtler because we usually don't know how it's supposed to look. When I do notice this in video, it's through faces and skintones, and it's honestly pretty distracting.
The fact that this effect still triggers while plugged in suggests to me that it isn't purely a power saving "feature," akin to what Intel mobile iGPUs have done for years on battery power. I also don't think it's an intentional effect, as Apple has prided itself on audiovisual quality and like I said earlier, these sorts of enhancements (at least, without user control) are generally frowned upon. So, best case is that it's just a side effect of moving to a new hardware decoder, and simply went unnoticed - as many in this thread have proven is very easy to do - and hopefully, can be fixed through a patch.
I've already sent out emails to Anand at anandtech and Displaymate prior to seeing this thread over the weekend, and received replies that they'll keep an eye out, so we might see actual coverage of the issue soon.
