The aspect of Nix which is relevant to "versioned OS" is the package configuration database which can be snapshotted (http://lwn.net/Articles/337677/ ).

In this approach, the static parts of a configuration —software packages, configuration files, control scripts— are built from pure functions, i.e., the results depend solely on the specified inputs of the function and are immutable. As a result, realising a system configuration becomes deterministic and reproducible. Upgrading to a new configuration is mostly atomic and doesn't overwrite anything of the old configuration, thus enabling rollbacks.

I recently wanted to install the same configuration as my Ubuntu laptop, on a desktop. The thread to list manually-installed packages in Ubuntu is huge and has no deterministic answer. Perhaps, a manager like Nix could have accorded me this flexibility.