Wayland isn't "just" a compositor, though that is a lot of what is does.
It is also an input demultiplexer, so keystrokes and mouse and touch and whatever can get to the right client.
It might even play a role in the IPC needed for cut/paste etc, I'm not completely clear on that.
But you are right in that wayland is just part of a complete solution. It is just a specification (I think).
You also need an implementation (like westron).
And you need toolkits to render images for the compositor to composite
and you probably need other bits. Maybe even a network transport - you could use X for that or maybe something new and better.