Re "it is a lisp" and "everything is an expression", I would like to add a bit of clarification.Or, given that you use Mathematica regularly while I was just reading surface docs (for purposes of doing some stuff with Wolfram Alpha), rather a question if my perspective is well-founded.
Based on my understanding of how expression evaluation works, the slightly more revealing statements would be "it is a lot of lisp macros" and "everything is an s-expression". Which means, a big mess. Let me expand:
As a functional programmer, "everything is an expression" sounds comforting, and I would expect there are clear transformation rules on how expressions are evaluated (and, maaybe, type signatures).
Instead, what you get is, "you can throw in some random form of expressions into this function, and it will do something with them". As in, it takes an AST input, and transforms them in some loosely specified way. There doesn't seem to be a type system, so you don't have types to guide you, rather you likely need to figure what kind of expressions work with which functions.
Now, if I'm wrong about this, and the functions behave consistently in what they take and how they transform it, then I'm more than open to be corrected. It is just that my high expectations (based on marketing of the lang) and the subsequent realization left me a bit bitter.