One thing that's often overlooked with Nix (the language) is that it does not let you define new data types. For example, there's no "package" type in Nix and Nixpkgs; instead there are only functions that are passed "attribute sets" (key/value dictionaries).

That Nix can't tell what's a package and what's not is a hindrance for its user interface (e.g., it's hard to look up packages by name/version or to iterate over them) and for packagers (e.g., easy to end up with package definitions that don't follow the informal "schema").