Clear visibility and contrast for all UI elements. Consistenty throughtout the system. Scrollbars that stay visible. Borders of windows clearly visible. Clear differentiation between buttons, checkboxes and radio buttons. No useless animations that make you miss clicks, take you out of the flow. Predictability and reliability of all UI components. Big icons designed for recognizability, not to look good on a screenshot/presentation.
In Windows we used to have themes... Wouldn't it be nice to be able to still customize window border thickness given our large resolution screens and precise mice? Now the task bar is huge, but can only be a single line, there is never enough text on task bar icons, windows explorer taskbar icons can't be reordered, yet if you try to quickly select an explorer icon in the taskbar, the entire group of windows explorer windows offers to be moved. We have rounded corners for the windows, but to resize diagonally you have to find the pixel where corner would be if the corner weren't rounded.
You know what takes the cake: In VSCode Nuget Gallery the search field is now entirely invisible in light theme. You literally have to click on the white space next to the filter button to reveal the search field. This his how it looks on my machine:
We still don't have a full and polished NeXTSTEP replicate after so many years. The closest thing is probably https://github.com/trunkmaster/nextspace