This pre-release includes several deprecations, new features, and JRuby optimizations. The most important changes are to thread pools. Thread pools have been completely rewritten. The behavior of FixedThreadPool and CachedThreadPool have not changed, but they now both extend a common base class called ThreadPoolExecutor. On JRuby this base class is a thin wrapper around java.util.concurrent.ThreadPoolExecutor. Subsequently the global thread pool has been updated from an instance of PerThreadExecutor (formerly NullThreadPool) to an instance of CachedThreadPool. This should increase the performance of all tasks using the global thread pool (such as Future, Agent, Promise, and others), especially on JRuby.
Please see the release notes for more information.