"...users no longer have to rely on package maintainers remembering to specify a specific version of a library or later -- instead, the needed version is installed as required. Just as importantly, NixOS avoids the problem of having the installation of the latest version of a key file added as a dependency for one application disabling another application that requires an earlier version. If a problem emerges anyway, then you can easily rollback the change by deleting the last installed program without danger of collateral damage to the rest of the system."

You can have different versions of the same applications, do atomic upgrades and rollbacks, allow safe non-root application installations, and do non-destructive upgrades. It's quite intriguing, and maybe someday we'll see Nix or some of its advanced concepts used in mainstream Linux distributions.