I believe its IDE was also the first widely used one to take the "mathematical notebook" approach where you can sort of free-form arrange formulas on a page and

I believe its IDE was also the first widely used one to take the "mathematical notebook" approach where you can sort of free-form arrange formulas on a page and link them to each other. Unless I'm mistaken, at the time the other computer-algebra systems (Macsyma, Maple, etc.) all used REPL style sequential interfaces. I mostly prefer the REPL interfaces myself, but a lot of people like the notebook style, and I think Mathematica's approach to that has been influential on some recent open-source interfaces like Sage Notebook and iPython Notebook.