Elisp is homoiconic and has a strong culture of REPL development, but really the thing that makes elisp so nice is that it has decades of use to iterate on an editor. There's an attempt to do this with Common Lisp that's quite cool, called Lem, but the sheer amount of person-years that have gone into making emacs and elisp a conjoined system makes it a pleasure to use and iterate on. Lem, in comparison, is building up all the UX associated with a "live programming environment" that just comes out of the box with elisp (though I, personally, prefer Common Lisp as a language/environment.)

Pre-LLM I used to spend the occasional afternoon at work when I was a bit demotivated just adding new features to my emacs or fixing small papercuts in the environment. Now I largely have the LLM work on it for me, though I separate out LLM code from my own by loading them separately in my config.