Nix is objectively a bad language. Glancing past the syntax (which is atrocious and the parser is completely unhelpful) it's just a bad idea to mix lazy evaluation and dynamic typing, the only way to get understandable error messages with nix is to put asserts all over the place, hoping one of them gets triggered by the evaluator.

That said I still love the system to bits and fixing the language actually wouldn't be that hard: Add static typing, with inference most stuff should just run as-is. Exception are the two or three uses of the y-combinator (yes, seriously, that's how you recurse in nix) deep in the bowels of nixpkgs, e.g. during stdenv bootstrap. Just make fix a primitive, or maybe a plain fold I don't think nix even needs to be Turing-complete. Reason it's not being done is that there's enough other stuff to work on and, well, Stockholm syndrome. Good news: Nix doesn't support constructing import paths dynamically, that would have been a nightmare to sort out in a static setting.

Yes, all the environment variables are set automatically and programs respect them.