Could you clarify what you mean about Nix missing concurrency and parallelism? I often run builds using nix-output-monitor and it definitely looks like things are running in parallel, although I could be mistaken.
I meant as part of the language itself, not just the runtime or for specific parts. Say I'm processing 100 JSON files, it'd be great if I could fire that off wrapped in 'parallel' or whatever, similar to Clojure and others I guess.