Two things spring to mind, there are lots more I'm sure.

Racket's macros are unlike any other. They are better thought of as a lightweight compiler API. Check out http://www.greghendershott.com/fear-of-macros/.

Also, the whole #lang framework is a pretty unique tool for experimenting with and extending the language, see http://docs.racket-lang.org/guide/hash-languages.html. For example, used to build http://www.greghendershott.com/rackjure/.

Greg Hendershott does some great writeups, there's more to find on his site.