Yeah, I agree. In the meantime there is an awesome re-implementation in Python called Mathics.
It's also fun to hack some of the not-yet implemented Mathematica commands (I did that last summer).
Mathics maintainer here.I'm glad you find Mathics fun to contribute to, I certainly do. Each function is a small independent part so it's really easy to get involved (that's a major goal of mine at least) Thanks for your contribution.