as far as i can tell, a good number of things not supported are intentional because of a different (better?) security model, and at least a few, like not supporting "window always on top" are simply false.
discuss away!
Maybe there are good reasons for Wayland to be the way it is, but my experience is that it's unnecessarily limited, it's in the way and "the rest" doesn't have these problem (Windows, Xorg, ...) so I'm not particularly fond of Wayland, even if it does a lot of things fine and it is quite stable (in my experience).
Other parts are things that are on the list only because X did it that way in 1985 - nobody actually does that in X but you can find a line item and scream that Wayland doesn't do that.
Most of the rest are things that can be supported but need a some more work, and progress is being made. Most are things most people don't even need/use - though of course if you are the rare person who uses them wayland won't work for you.
The remainder is NVIDIA who is way behind. We have known they are not a good linux citizen for more than 15 years now though, so I can't have sympathy for those who buy their products. (yes I know they are the fastest in other ways, but their linux support as always been a hack)