The Wayland Situation: Facts About X vs. Wayland (Phoronix)

Multi-display will work without any changes in applications right now. After all, applications won't care about who's going to be displaying their buffers with pixels and the compositor can send them to whatever displays it wants.

But if you are going to do something more complicated, like switchable graphics, then you're going to have to support it within the applications. It'd be nice to have it on the Mesa (OpenGL) level, though.