This page tracks the Wayland showstoppers through out the stack
No vertical / horizontal maximize of Wayland native clients
KWin uses NetWM modes internally to do this. Works with Xwayland clients. Would also work for Wayland clients, just the state cannot be exported to Wayland (c.f. Quick tiling).
No remote support
We don't have an API yet for remote support. Even if we had things like TeamViewer would not work any more.
Partly fixed since we now have an API for screencast using a separate KWayland interface. There is also already support in xdg-desktop-portals for convenience of other apps and general support in Flatpaks. What's still missing is an API for remote input.
The NVIDIA blob uses a custom EGL extension which Plasma 5.16 added support for. It requires setting the KWIN_DRM_USE_EGL_STREAMS=1 environment variable to activate.
Other aspects of the "Wayland experience" with NVIDIA such as XWayland compatibility are outside of our influence.
Partly fixed: Requires manual activation.
No Clipboard Manager protocol
No clipboard history (e.g. Klipper and clipboard plasmoid) and no clipboard sync with KDE Connect.
In progress: https://phabricator.kde.org/T4449
No (wacom) Tablet support
KWin lacks integration the tablet support from libinput. Also QtWayland and KWayland lack wrapping the API for it.
No configurable gestures
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.
Native Wayland windows are not restored
Session restoring does not include Wayland native windows. Our sessions management recover engine is based on the X Session Management Protocol and there is apparently currently no generic concept to do it on Wayland. But on Qt it's plugin-able and GNOME has according to their wiki an own implementation for some time.
No window thumbnails
We don't have an API yet for window thumbnails.
No window move on empty area in breeze
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.