Maturity? Gets out of the way? Advanced documentation/learning material? Wealth of prior work solutions to common patterns? It's not so hard to guess what any developer's productivity is hinged on.

I asked because many people that used them called out the machines as being very productive...above and beyond lisp. Another comment here suggests it might have been CLIM.

Ah. A lot of that was because the LispMs had really good IDEs at a time when IDEs were all but unheard of. This was possible because it's a lot easier to write an IDE for Lisp than for C. But all that can be re-created at the application level (obviously because IDEs are applications). There's no need for a system-level Lisp to get that win.

In fact, Clozure Common Lisp is a perfect example. It provides (IMHO) 80-90% of the productivity advantages of a LispM when you run it on a Mac (because the IDE is Mac-specific).

I'm intrigued too, what's the 20-10% that's missing that was present in the LispMs?

Mainly the ability to write and debug system code with the same ease as you debug application code.