The Eureka moment for me was when I realized that I can express LET in terms of LAMBDA. And why stop there? I can express DEFUN in terms of LAMBDA too. All of the code is just LAMBDAs and LAMBDAs. That's when it really clicked for me!
When I discovered that I could forget the cons, car, cdr of LISP and rebuild them from the ground, the lambda-calculus, a very basic "text rewriting" tool, the Swiss knife of the code, the TAO of code ...