Honestly that's the only thing that keeps me from jumping to NixOS. I don't want to learn a weird language, I'll never use for anything else, just to maintain my OS.
The language is so simple you can learn all of it in a day. Read through chapter 15 in the nix[1] manual and that's it.
To me it doesn't feel weird at all: I can't think of any feature that was new or surprising when I started using it and I am not the biggest expert of functional programming. You have to learn more about the standard environment and tools if you want to start contributing to nixpkgs and write your own packages but if you only care about configuring your system nix is as difficult to learn as say, JSON.
Personally, I'm not against the idea in principle. There are some real DSL-haters out there, and I'm not one of them. For a different problem domain, a different language can be more efficient.
It's true that some DSLs are not only different, they're also badly-designed languages or gratuitously different. I guess this is an occupational hazard of DSLs, but it's not unavoidable. It's possible to make the leap to a DSL only when actually beneficial and then to design a DSL that doesn't suck.
To me it doesn't feel weird at all: I can't think of any feature that was new or surprising when I started using it and I am not the biggest expert of functional programming. You have to learn more about the standard environment and tools if you want to start contributing to nixpkgs and write your own packages but if you only care about configuring your system nix is as difficult to learn as say, JSON.
[1]: https://nixos.org/nix/manual/#ch-expression-language