I think another "overlooked" feature of java is the JVM itself. It is probably the best VM available. And it supports lots of interesting and useful languages (Jython, JRuby, Scala, Groovy). All those languages can easily and seamlessly cooperate.
If you design a new language (like in the scala-case) you immediately have all the existing libraries available and your language is therefore "useful" from the very beginning.
All those languages make use of the HotSpot optimizations. The VM is very well monitor and debuggable.
