What makes Lisp...Lisp?

If you starting removing features from Lisp, at what point is it no longer Lisp? Some people claim Clojure shouldn't be considered a Lisp. Is it because too much Lisp was removed?

What is at the very heart of Lisp that makes it Lisp?

>>29
MAPCAR is more generic than that, but even assuming you're fine with such a definition, you're going to have to write a lot more specific boilerplate code than you were going to do in Lisp, just look at that size_t.