[fijal]

I claim you're wrong. I've been working on a range of JIT compilers, but the PyPy is the biggest one. The problem is not "how hard the semantics are", but "how much of the semantics you need to support", especially that interaction between them is a headache. Lua does not have the equivalent of where statement in JS (albeit most JIT compilers just don't optimize it) for example.

I won't argue about the JS (although DOM interaction comes to mind as a big headache), but in Python syntax is trivial. It's all the semantics and not just how they're, but how much of it. descriptors, metaclassses, new/old style classes, tons of builtin types, tons of builtin modules, all of it the user will expect to seamlessly integrate with the JIT compiler.