The "two language problem" in the high performance numerical computation domain, in which you have a high level language that is easy to use as a glue (such as Python) and a low level language for anything that requires performance (such as C++). In Julia a program that looks like python can run at the speed of C (and as a bonus has the metaprogramming abilities of a Lisp minus s-expressions), and with only one high level language for everything you can much more easily create, understand, extend and debug anything.
