Many incremental changes (some already mentioned here) but none comes close to grokking macros in Lisp and lisp-like programming languages such as Common Lisp,

Many incremental changes (some already mentioned here) but none comes close to grokking macros in Lisp and lisp-like programming languages such as Common Lisp, Scheme, Racket, Emacs Lisp and Clojure. That was truly, truly transformative.