NixOS needs what is IMO the killer feature of Arch: the wiki.
Comprehensive documentation on not only the OS but the additional packages that we use is what drew me to Arch, and the thing that makes me swear in frustration whenever I have to use Ubuntu/Debian.
NixOS is an excellent OS that has the promise of being every bit as hackable as Arch, but far more stable. Problem is, configuration is very different and needs extensive documentation to reduce that friction point.