The main feature that Clojure provides for concurrency is immutability, not just the possibility of using existing data structures in an immutable manner, but an entire suite of immutable data types and data structures. This makes concurrency, even using Java's primitives, like Thread, (which was the original way Clojure was intended to be used anyway,) with Clojure's data structures is a win over using Java's if you want safe, predictable behaviour in a concurrent program.