I'm doing occasionally some cross-platform development using Windows API and X11 and what I hope mostly for is that Wayland will reduce the complexity when programming compared to what is needed for X. Even simply things get complicated on X11 due to the client&server communication. A simple call to check the cursor-position can freeze your application for several milliseconds when the server decides to ignore your client for a few frames (and yes, depending on the application that matters - for games it's really annoying). Or adding copy&paste support will send you through a hell of no-longer-working examples and you can find lots of custom implementation where no 2 people tried the same workaround to ensure they really wait long enough for the server to respond and filter the right messages. I think the X people are also working on improving the situation when client&server are on the same system, but unfortunately you never can really ignore that communication completely and it will probably be many years before older X11 systems out there can safely be ignored.

I realize many people care a lot about the network part of X11. I suppose especially if you are working as Sysop it might be a big deal. I've not used it for over a decade, so obviously not something I care about so much. But on the programming side that client&server split makes life a lot harder and is not a solved problem.