Rich Hickey has gone to great lengths to define simplicity and complexity (http://www.infoq.com/presentations/Simple-Made-Easy), and the paper "Out of the Tar Pit" (http://shaffner.us/cs/papers/tarpit.pdf) goes further.

Software projects are becoming increasingly complex, and multicore concurrency is becoming increasingly important. Immutability and referential transparency are key ingredients in Clojure concurrency so I think the slogan is apt.