1) Wayland is really slow. I don't know if it's the compositing or what but it's unusable on lighter hardware that X ran fine on.
2) Widget toolkits handling window decoration is awful. Before the large number of toolkits just meant some controls were a little different but now basic behavior changes based on how programmers decided to build an app. And if you don't like the window decorations (say, they take up too much screen space) your choices are suck it up, or if you're lucky and willing to spend a bunch of time reconfigure every different toolkit your apps use.
3) basic stuff that worked fine on X11 doesn't work on wayland in the name of "security" (screenshots are a big one, there are extensions but isn't that the complaint about X? And if there's a security problem with something isn't hacking around with it because people need it a really strong indication that the idea is broken and probably making the situation worse?)