Learning the concepts and ideas behind lisp really expanded my horizons as a programmer. For me, that all started with emacs. Though elisp isn't very fast and has some pain points, emacs as a whole gave me an extremely established customizable code base that I could play around and learn in.

Though it's cliché and often listed as a negative, emacs really gives the user more than just a program to edit text.

I don't have anything against Lisp, I program Emacs with it, but wouldn't you get mostly the same features if Emacs used Javascript for its programming, for example?

[0] https://github.com/emacs-ng/emacs-ng Disclosure I've posted about this on HN in the past, but I felt it was very relevant to the parent comment.