> Multiple monitors are actually supported by the protocol.
They're not really supported, just partially exposed. To configure them they must be supported by compositor. Userspace tools can no longer switch/move/rotate monitors. For X.Org you can use xrandr tool anywhere. For Wayland you'll have to use compositor-specific tool, if it exists. And you can't write a presentation tool that will automatically rotate monitor to portrait orientation and back, unless you make it part of the compositor.
> The other bits you've called out have no effect on clients, so they aren't supported in the client-server protocol.
"Client" definition in Wayland is different from X.Org. For X.Org all visible apps on the screen are X.Org clients. And most of them could never be Wayland clients. Under Wayland same apps have to be integrated with compositor.
> Weston is totally pluggable
As long as there's no stable Weston API for dynamically loadable plugins, "weston plugin" won't be much different from "weston patch".