I don't think that's entirely unique to Lisps, you can get a REPL into an existing Erlang instance very easily. More easily than getting nREPL running in Clojure, actually. I believe Smalltalk as well, although I'm less confident on that one as I haven't used it much.
As someone who's professionally developed mostly in Erlang and Clojure in the last 6 years, it's not even close. I love Erlang, but the REPL is severely underpowered and used mostly for debugging and inspection.
The difference is in the editor integration. When you work with Lisp, your editor is connected to the running instance of the application. You have shortcuts to reload any symbol in the application at runtime directly from the editor.
