I think you could argue that type-safe STM is such a feature for Haskell. It's something that I really miss when using threads or async essentially anywhere els

I think you could argue that type-safe STM is such a feature for Haskell. It's something that I really miss when using threads or async essentially anywhere else. I would probably jump for Haskell over Rust as a result for heavily concurrent or multithreaded programs. Rust is memory safe, but it can't express purity and it can't enforce that STM transactions should be safely retryable. I would also argue that Elm offers close to a 10x factor for certain types of front-end web applications. It's a very closed system, which is a matter of great controversy, but in areas where you can exist within that system it is a dramatic leap forward compared to anything else in my experience. If you consider Erlang/Elixir functional languages, I would argue that BEAM can offer close to a 10x factor for certain types of distributed or concurrent applications as well. You mentioned Clojure, though, and I don't know what the killer 10x feature would be there.