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.