Programming language runtimes tend to be somewhat complex (real world ones, at least). The implementation of the internals of a modern LLVM-based JIT language is bound to be somewhat "esoteric" regardless of what it's written in – using C++11 wouldn't change that. The developer docs are pretty comprehensive these days and a significant number of people have "picked it up" and become serious contributors to Julia's internals. In fact, of the people who hack on internals the most these days, none are original developers. (I'm thinking of Jameson, Yichao and Keno for those who are familiar with Julia's development.)
