Despite all that, JRuby does an absolutely bang-up job of working around the JVM's limitations where it can. As far as I know, it was the first language implementation anywhere on the JVM to allow forking, for instance.
True, but forking is also something which most languages running on top of Posix can do, that's not saying much.
What I really want and what is needed - a reference implementation written in 80% Ruby (or more). That's like Rubinius or Pypy, but adoption isn't great even though these virtual machines have shown better performance than the reference implementation for computational-heavy tasks.
And adoption isn't great because remarkable libraries written in native code won't run reliably or at all on top of them (e.g. Python's Scipy).
And JRuby shares the same problem, although it has the advantage of being able to use JVM-alternatives. But then you're stepping out of the Ruby ecosystem, which makes it a fork.
And to get back to "JRuby as reference implementation" ... it would be an even greater pain in the ass for alternative implementations, because C is more portable than Java.
Oh, absolutely. I'm not saying it's perfect, by any means - just that the JRuby team have pulled off an astounding job to get it as good as it is, given the limits of the JVM.
Also, native libraries are becoming less of a problem now that libffi has a bit of momentum.
