The biggest for me is actual tear-free rendering. All the different hacks in Xorg have never worked completely. The other one that people value a lot is HiDPI

The biggest for me is actual tear-free rendering. All the different hacks in Xorg have never worked completely. The other one that people value a lot is HiDPI scaling done "well". Mixed fractional scaling in Xorg doesn't really work and it's finally possible to do it. Although there I actually think it's still a half step and Wayland made the wrong choice in how to implement it. It should have been done client side for more performance and less fuzzyness. There's a chance that will also be fixed in the future. Other than these it's a bunch of small things that are more technical than user-visible. Security is much improved for example. Some performance gains can be had from avoiding copying buffers. Etc.