On the top of my head:
- good IDE support (with refactoring)
- stable enough specifications
- clear policy about backward compatibility (in term of sources, binaries and behavior)
- frameworks and tools around the language (like code static analysis, code coverage, …)
Scala, for instance, is still evolving and moving “too fast” to be “largely” used even though some big projects have already adopted it.
Edit: November 2009
See Refactoring to Scala DSLs and LiftOff 2009 Recap presentation, Slide 10 and 11:
Martin definitively “gets it” about the importance of having high quality Scala tool support