mixed DPI scaling (can drag a window from a 1x screen to a 1.75x screen and have it look correct on both at the same time, even when halfway across each)
Mixed refresh rate (my center monitor is higher refresh rate than my side monitors, X11 just baselines all monitors to the lowest common denominator).
Mixed variable refresh rate (center monitor is VRR capable, side monitors are not).
HDR support soon (already exists in GameScope).
Mixed HDR/SDR output across monitors
Performance:
Lower resources
Smoother operation (can be felt in mouse cursor movements, window drags, composited animations, etc)
Better VR headset isolation compared to X11 (allows the headset to run separately and not interrupt regular monitor layout, and also lets it run freely at the correct refresh rate)
Other:
Better security between apps (yes I actually use this and count it as a feature)
App video isolation leads to pipe wire functionality, which is a bonus and makes OBS work better overall
I know for a fact I’m forgetting something because this list was longer the last time I wrote it out, but I think you get the point.