For me, the killer feature of Clojure is "REPL driven development". The ability to get rapid feedback as you build things up is incredible. I'll make a comment form and evaluate expressions within it to try things out as I go. By the time I've written any reasonably complex function, all of the pieces have been tested on various examples. Once I'm done, the comment form leaves a helpful record of my thought process.
If you're wondering what's so great about the Clojure repl (and other lisp repls) in particular, the thing is that you never have to actually type something into it. You can run evaluations from the code file itself. The structure of lisps makes it clear exactly which code you want to evaluate. To do this, I use the excellent CIDER package for emacs. I understand Calva for VS Code and Cursive for IntellJ offer similar functionality.
If you're wondering what's so great about the Clojure repl (and other lisp repls) in particular, the thing is that you never have to actually type something into it. You can run evaluations from the code file itself. The structure of lisps makes it clear exactly which code you want to evaluate. To do this, I use the excellent CIDER package for emacs. I understand Calva for VS Code and Cursive for IntellJ offer similar functionality.
Here's a good talk on what this looks like:
https://www.youtube.com/watch?v=gIoadGfm5T8