> Why is RPN a head trip?

It isn't how we teach math in the two countries I've lived in. Lisp is just as weird. Many people see a mathematical expression and panic. A level up from them, people see a mathematical expression in some source code and expect it to respect the symbol precedence that they were taught in grade school -- which they can "understand" without knowing how the language parses and abstracts all that away. And maybe their understanding is flawed but they can survive as programmers for decades without ever going deeper. Lisps, FORTHs, etc., don't allow you to proceed without understanding.