Not only the JVM (which is rock-solid at this point) but also immutability, persistent data structures, the separation between values and identities and its sol

Not only the JVM (which is rock-solid at this point) but also immutability, persistent data structures, the separation between values and identities and its solution to the expression problem which is protocols. I was at first turned down by the JVM (not knowing any better back then), but it turns out being a hosted language is one of the best aspects of Clojure.