The one that has been most frequently discussed was the lack of support in the beginning for screen sharing and the like, but there are many others.

X11 is such a central part of the software required to use a computer that it is not acceptable for a substitute to implement only a subset of its functions.

For any X11 replacement, its architecture should have been conceived since the beginning to enable the implementation of all X11 functions, even if for some of them a lower performance caused by interposed compatibility layers is acceptable, if it is hoped that they will be deprecated eventually.

It should have been obvious for the Wayland developers that some of the X11 features will never be deprecated, e.g. screen snapshots, screen sharing or remote desktop access.

Instead of trying to remove such features, they should have tried since the beginning to find better solutions for them then in X11.

While a network protocol should be implemented by a program distinct from the graphics system, when designing any modern graphics API one should ensure that it has good compatibility with something like the Remote Desktop Protocol.

In general the Wayland designers have attempted to minimize the work they had to do by claiming that various functions belong into other programs, but their arguments are not convincing.

Most, if not all, functions of a window manager should be implemented in the same process as the graphics system, even if the window manager should be some kind of replaceable plugin. That includes the window decorations. A GUI application must be concerned only with the client area of a window.

Moreover, it is inexcusable that Wayland has not been designed with an up-to-date color management since the beginning.