For example, it doesn't have an API to get current cursor position (which breaks keepassxc's browser popup, goldendict's query popup), an API (which most compositor implements) to get current window state, which makes me unable to find an alternative of autokey (kwin script can do this, though, but it lacks the ability to execute arbitary commands..).
On the other hand, I don't see a killing feature that drives people switch to wayland.. (HDR can be one, but I don't use it) I mean, yeah X11 is old and unmaintained, but it WORKS.