Too bad I can't bitmap-font UI elements. Being able to scale those freely (my DE allows it by a float value that is currently around 1,65) without too noticeable blur is my most important reason for a high resolution screen.
The by current standards normal sharpness is nice too (1080 screens aren't really blurry, they're pixely).
Examples:
https://github.com/Tecate/bitmap-fonts
If you're on 4K you can simulate how they look on 1080p with 2x integer nearest neighbor scaling.