The insanely steep learning curve, for something that manages packages. Nix will never take off until you ditch the DSL and get documentation in shape. Otherwise Nix will continue being a system developed by very smart engineers for very smart engineers.

I think the UX problems with Nix are well documented and understood. The developers behind it just need to focus. For that focus to take place, the project needs to be placed under better stewardship.

nix is a build tool, not a package manager. Misunderstanding nix as a package manager (and the dumb marketing on its website) is what leads to frustration (along with, of course, docs).