The most obvious, user-visible loss of features are applications no longer able to grab/mess with contents of another application’s window. Screen sharing and remote desktop was broken for a long time in wayland until it’s fixed via pipewire recently. Under X11, rendering is free-for-all, where any app is free to do whatever it wants to other app’s window. Heck, you can even tell mpv to play video on a cell in librecalc if you feel like it. Such shenanigans is now impossible in wayland because it’s a big security risk (though I’m not sure if it’s actually exploited in the wild).

The most hyped feature of wayland is better support for high resolution “retina” display. Also, you can use multiple monitors with different dpi/scaling in wayland. IIRC it’s not possible on X11, though you can use xrandr to force the scaling on each monitor, though it’ll result in blurry texts because the scaling is not done natively.