I'm inclined to think the Clojure REPL is more conducive to flailing around. Now, this might sound like derision, but it isn't intended that way: Flailing around can be a very time-efficient way to familiarize oneself with an unknown domain, especially if the cost of making wrong design decisions is small.
But at some point one has to consolidate what one already has, and, in my experience, Clojure makes this very difficult.
> I guess one programmer's sandbox is another programmer's hellish nightmare of mutability.
If you absolutely want mutable definitions, you can have them in Haskell (with some noise) and ML (with no noise) too: http://pastebin.com/00ScnFxC . So a Haskell or ML programmer always has at their disposal whatever they think is the best tool for the job.
Can I have it the other way around in Clojure?