Disclaimer: I don't know a ton about this. But one of the often-cited reasons for learning a Lisp is the power of Lisp macros for creating domain-specific languages. My understanding is that one unique feature of Racket is how it takes this idea to the next level: not only does it offer a powerful hygienic macro system; Racket also thinks of "languages" as libraries. The Racket folks have built a platform where it is easy to define non-trivial (and even non-Lispy) languages, and freely incorporate code written in those languages into other Racket programs.