The expr is evaluated and thrown, therefore it should yield an instance of some derivee of Throwable. Please see http://clojure.org/special_forms#throw
=> (throw (Exception. "my exception message"))
java.lang.Exception: my exception message (NO_SOURCE_FILE:0)
;; Different types of exception can be thrown
=> (throw (AssertionError. "Wrong input."))
java.langAssertionError: Wrong input.