Beginner level blogposts (that are plenty) talk about the same few features like ad-hoc development shell.
I think it's not so much a problem of Nix/NixOS, but a consequence of the power of composability. For the next n years, people will keep discovering new things you can do. As the knowledge spreads, people will work out how to document the features and present them in ways that can be easily grasped. But the final success of NixOS will be when every distribution adopts more and more NixOS-like composability and NixOS is well understood and even boring, like the final success of Unix.
OTOH it finally got me over my fear of being a n00b and just use the source.
But I am extremely faint of heart, and I love NixOS.
The trick is that I'm not a developer, and I happen not to need to ask it to do anything complicated or non-standard. I expect lots of other people are in my position (although maybe not many of those people read HN).