This page tracks the Wayland showstoppers through out the stack
The NVIDIA blob is not supported as it uses a custom EGL extension. It would require additional code just for NVIDIA. On the other hand many users are on NVIDIA. Further information: To EGL Stream or Not and Plasma/Wayland and NVIDIA – 2017 edition
XWayland also uses the DRM/GBM mechanism. Due to that XWayland is not supporting OpenGL on NVIDIA even if KWin would support NVIDIA's eglstream. This would result in a terrible experience for all gamers.
QT_QPA_PLATFORM is set to wayland breaking any application which has Qt included (e.g. appimages, proprietary software) and doesn't have wayland QPA.
We don't have an API yet for remote support. Even if we had things like TeamViewer would not work any more.
KWin lacks integration the tablet support from libinput. Also KWayland lacks wrapping the API for it.
X has the ability to accept fake input events to allow programs such as Easystroke. With the new security model of Wayland, such a program would need to be part of the compositor.
We lack a functionality like XModmap to remap keys.
We don't have an API yet for virtual desktops, so pager is not working
We don't have an API yet for window thumbnails.
Broke with switch to XdgShellV6. Needs new API added to Qt (native interface), KWayland and Breeze.
Activities are probably not a must-have feature, but anyway this is annoying for those who got accustomed to them on X11.
QtWayland does not yet support the tablet API. So even if KWin had support Qt would not be able to use it.