I personally don't like fast changing languages, they have the air of incompleteness about them. |
It really depends on the development process and priorities made by the team evolving the language. I have more feel of incompleteness about Java and C++ than about Scala or Clojure.
The latters are mostly adding new library features, not changing the language core (especially in case of Clojure which is a LISP variant, but Scala's OOP is also a lot simpler than Java's). Having a simple, consistent and extensible core set of language features is what decides whether it is easy to evolve a language or not.
It is far easier to change a library than to change a core lanuage feature. Many things that are core language features of Java and C++, are accomplished by libraries in the faster changing languages.
If you put into the core language all possible features that some of the programmers would like to have, then it would be very hard to make all of them orthogonal and quickly you will end up with adding new features just to fix the problems with the old features. Also adding completely new things, that in isolation seem to be very simple, will be very, very, hard. This is what actually happened in C++0x, e.g. with concepts - they had to leave them out because of lots of corner cases they caused. While other languages have a similar feature for many, many years, without problems. Another example are perfect generic forwarder functions - problematic only in C++.