For me, it’s Shared GPU memory.
Online games just working.
I know, I’m probably off better without Chinese and Korean rootkits installed, but Infinity Nikki looks so darn comfy to play.
Oh, and HDR and 144Hz. Both in X-Server as well as in Wayland, over a good DP, I can select 199.98Hz at best. Never managed to fix it. Same computer, monitor and cable used to do 144Hz just fine on Windows, before.
HDR is really gone, though, but I don’t miss that as much.