I was never a user of PyPy but I really appreciated the (successful) effort to cleanly extract from Python a layer that of essential primitives upon which the rest of the language's features and sugar could be implemented.

It's more than just what is syntax or a language feature, for example RPython provides nts classes, but only very limited multiple inheritance; all the MRO stuff is implemented using RPython for PyPy itself.

The key difference is that RPython is actually a compiled language.

I.e. PyPy DOESN'T have an interpreter written in an interpreted language.