There's a person above saying that it's about being to able to mutate program state from the repl, which is a thing that's also possible in any repl for a language with managed memory.
There are some other affordances for interactive programming, such as a standard way to update existing instances of classes. I’m sure you could implement this sort of functionality in any language, but this is universal and comes for free in Common Lisp.
CL also has other interesting features such as macros, multiple dispatch, compilation at runtime, and being able to save a memory snapshot of the program. It’s quite unique.
To my knowledge there’s no real “mainstream” language that goes all in on interactive development. Breakpoints and traceback are all perfectly cromulent ways to debug, but it’s really not the same, sadly.