REPL. homoiconicity makes "real" REPL possible. Anyone who hasn't tried Lisp would say: "a lot of other [non-lispy] languages have REPLs". But only after trying Lisp REPL you can truly experience why is it so special. It is extremely liberating to be able to evaluate any part of your [running] program, anytime, [almost] without any preceding ceremony. Even famous Jupyter Notebooks fall short in comparison.