What is missing?

There are lots of ways in which Mathics could still be improved.

Most notably, performance is still very slow, so any serious usage in cutting-edge industry or research will fail, unfortunately. Speeding up pattern matching, maybe "out-sourcing" parts of it from Python to C, would certainly improve the whole Mathics experience.

Apart from performance issues, new features such as more functions in various mathematical fields like calculus, number theory, or graph theory are still to be added.