I) “Every frame is perfect.” Wayland's main goal is that no matter the system load, no matter what is going on, it will be flicker free, tear free, and flash free. Every frame is presented in the correct and proper order (dropping frames is fine, but you wont get frame 199, followed by 205, followed by 200 because they all got sent at roughly the same time and the server picked them at random). Wayland knows what order they came in, what order they need to be displayed in, and knows WHEN they were displayed because EVERYTHING has a timestamp associated with it.
II) Minimal!!! We've learned the hard way what happens when you have something do a lot of things that also has to maintain backwards compatibility-- 26yr old mistakes are still biting us in the ass TODAY with X. Let the clients handle things, they can change-- they can break things all they want because its THEM who has to deal with the fallout of that breakage. We're helping to future-proof Wayland by reducing the surface-area for mistakes.
III) Hardware specific backends. I'm sure some people saw that the Rasberry Pi got a Wayland specific backend, and how it allowed the hardware to be taken advantage of more fully. It wont be necessary for all things, most things WONT need a hardware-specific backend...but it sure its nice to have it available. It means we have freedom, we have choice to make specific tweaks if we need to. Or if we realize down the road that the main backend has some architectural flaw in it, we can change it out with one that doesn't.