Why is #Wayland preferable to #X11? I honestly haven't seen a good argument yet. I haven't looked for a good argument either, but usually articles about Wayland just go on explaining how it will accomplish goals, but no one I've read has explained why they want it.
@poetgrant X is a messy hodgepodge. Wayland can dump a bunch of legacy stuff and take over (thus unifying) some functions, e.g. compositing. It's less client-server oriented, which might be a better reflection of how X is used these days.