Yep, the Wayland developers' solution to window management is for anyone who wants a different window manager to replace Wayland with a different compositor+window manager speaking the same protocol. Their suggested way of supporting proprietary graphics drivers is also replacing the compositor+window manager with one specific to that hardware which speaks the Wayland protocol, sort of like in the early days of X when there was a seperate X server binary for every piece of hardware. So your window management choices under Wayland depend on which graphics drivers you're using.
Not quite. Wayland itself doesn't require any graphics-driver-specific code; it uses the standard kernel interfaces and graphics stack (Mesa, DRI, libdrm, KMS, etc). That also works fine with radeon and nouveau, and it would work fine with the proprietary drivers as long as they support the standard graphics stack. It just won't work with proprietary drivers that implement their own entirely un-integrated graphics stack, which seems unsurprising to me.
Your window manager of choice can just link into Wayland/Weston and ignore all the underlying graphics driver bits; they'll Just Work.
It's been years and years since I knew the details, but at one point the nvidia drivers replaced pretty everything from libX11 down, and I don't think you could hook in at a middle point. (ie, they don't support the "standard" graphics stack.) Would appreciate if anyone could comment on how things stand today.
Somewhat better, but still awful. They don't replace libX11, but they do replace libGL, and they bypass the entire kernel graphics stack in favor of their own.
While its true that a new "WM" in wayland would be a diffent binary its not really the same as a different X server. All the stuff that X does wrt hardware and stuff is not really in the wayland compositor, but in the drivers, and they are the same with both WMs.
And the WMs would link to the wayland server library, so a lot of code would be shared between them.
I don't think properietary drivers need to have their own wayland server. However, they do need to add EGL support, so that wayland can use the driver outside of X, and a few specific GL extensions that wayland needs. See e.g:
http://ppaalanen.blogspot.fr/2012/03/what-does-egl-do-in-way...
