In Mathematica, the equivalent of a "function call" would be something like f[x,y]. I can write and run that, and it won't give an error if f, x or y aren't defined. Fundamentally, all atoms in the language are terms of symbols. Expressions are broken down by patterns until they're reduced as far as they can be, then the language happily stops and returns how far it got.

It's like Lisp, if everything in Lisp were quoted, and eval consisted of pattern matching and applying substitution rules.

As an example, say in Haskell I have foo x y = x + 2/y. Can you write a function that takes any function in, and replaces all + with *? in Mathematica you can: foo[x_,y_] := x + 2/y; foo[x,y] /. l_ + r_ -> l*r.

I'm sure you can do this stuff in other languages if you try hard enough. Some kind of reflection in Haskell or F#. In Lisp, grab and quote the definition of foo and apply macro machinery to it. But that's not the tao of those languages, while it is the essence of Mathematica.