Clojure encourages functional style with persistent data structures, a rich library of pure functions, and powerful processing support via the seq and reducer abstractions. Clojure implements a reference model for state, where references represent atomic successions of values, and change is encapsulated by value and reference constructors. This reference model is more substantive and suitable to application development than individual techniques such as Software Transactional Memory (STM) or actors.
The most important single principle behind Clojure is simplicity. Clojure's abstractions are simple and orthogonal. A la carte polymorphism, careful support for names and namespaces, the reference succession model, and a wide selection of small, composable protocols make Clojure programming swift, surgical and accurate.
Clojure's expressiveness does not mean that you have to compromise on power. It is an explicit design goal of Clojure to provide access to the power of the underlying platform, and for programmers never to have to "drop down" to the platform level for performance-sensitive work.