I do think it's important to emphasize the difference between installing software in user environments and system wide, which is why the tool is named nix-env. System packages must be installed via the nixos configuration file and a rebuild.

To me the biggest missing piece for new users is a tool to help manage your system configuration and reduce the frustration of having to constantly look up nix syntax or nixpkgs quirks. Maybe thinking of it as a nixos/nix IDE? Also a polished distribution built on nixos would be a good starting point (and easy to do, I didn't realize I could just copy someone else's configuration until well after I had my system working well enough)

The other thing that got me starting out is the need to garbage collect old packages. It's not strictly necessary if you have a large enough disk, but it took me several iterations of filling my root partition before I figured out how to properly clean up old generations.