Guix System. The way that this distro keeps track of changes of the distro itself. The concept of having a store where everything you build is stored there with write protection. The fact that you can configure not only the system but every home environment to every detail but without having to deal with various configuration files that you keep track of it.
The fact that all builds are bit by bit reproducible. The extensibility you have in your system.
It's the first distro I feel that nothing in your own OS instance is tied to any distro decisions.
The fact that you can have multiple versions of the same library without breaking the system.
It has a lot of things that I never thought it could be possible with a distro without going crazy about creating a very messy configuration.
I come from Debian stable so...
I'm currently ending the Guix manual. I want to add freetube and N64recompiled packages. Didn't know it's difficult to get patches or packages update to mainstream.
It's a bit funny that the records that Guix uses are not the baseline records of the Guile api but modified ones. And the documentation in some low-level regards is scarce.
But using Guix opens up endless options and more importantly it helps you manage and learn how to setup operating systems.