NixOS's package manager Nix allows the creation of project-specific shell and build environments which are isolated from the rest of the system. These environments are defined declaratively to ensure reproducibility.
Atomic non-destructive upgrades/rollback of a system upgrade/declarative reproducible system configuration/unprivileged installation of packages/transparent source or binary deployment.
NixOS is configured using the Nix package manager, allowing your system to be replicated and kept in sync across multiple machines. Great for keeping a laptop and desktop in sync.
A lot of the documentation of various functions is buried on the source code, their respective manuals, or non-existent. The documentation, the conventions, and the scattered toolchain really made searching for stuff easily missable.