tl;dr: Lisp's power comes from the combination of homoiconicity (code-as-data), macros that let you extend the language itself, and a live REPL-driven development environment where you continuously evaluate and redefine code in a running process. These features enable programmers to build domain-specific languages tailored to their problem, and make software inherently extensible—users can leverage the same DSLs the developer used internally, as seen in AutoCAD's AutoLISP and Emacs. Though Lisp never became mainstream, learning it fundamentally changes how you think about programming.