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?

>>34
By the way, that is incorrect:

(cons 'a (cons 'b (cons 'c nil)))
Evaluates to (a b c), not '(a b c) which is a literal list. CONS provides freshly created cons cells.