Loop, like structured editing (Paredit), is one of those Interlisp things that's very controversial and divisive.
There's tons of really wild ideas in Interlisp that seemed to be the half-baked acid trip ideas of West Coast hippies at the time, that are just starting to become rediscovered in the past couple of years (pervasive undo -> reversible debugging, DWIM-like autosuggestions in more places), and even the implementation techniques used are still innovative (for example the error-trapping implementation of Conversational Lisp (http://docs.google.com/viewer?a=v&q=cache:4GnEnGS2XXkJ:c...) is quite similar to how Geoff Wozniak approached auto-defining functions (http://exploring-lisp.blogspot.com/2008/01/auto-defining-fun...)
I'm going by the Hyperspec and what I remember from reading Kaisler's Interlisp. From the former:
"One of the Interlisp ideas that influenced Common Lisp was an iteration construct implemented by Warren Teitelman that inspired the loop macro used both on the Lisp Machines and in MacLisp, and now in Common Lisp."