document updated 14 years ago, on Dec 4, 2009
The biggest thing that holds modern programming languages back is the lack of a VM that can do more flexible things (first-class continuations, fibers/coroutines, tail calls, etc).
It is difficult to go back and re-engineer a VM to add these features. Without these features, these VMs can never efficiently run programming languages that use these features. (despite the Java VM hosting a vast variety of programming languages, it'll never be able to do many of them efficiently)
VMs that definitely will never have these
VMs that already do these
VMs that are moving in the right direction