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Something that gets attention from Java developers is Scala's implementation of the Actor concurrency model, which is a fundamental alternative to the shared-state locking that Java provides. Erlang has offered this for many years, but Scala offers it with a more familiar syntax, and is likely to get a more enthusiastic uptake for that reason alone.

As well as this Scala is functional (first-class functions, currying etc) as well as OO, and Scala programs can use the standard Java libraries if they wish, which again makes the transition easier for Java developers.