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?

Number six there make for some nice top-down design - but I never knew the tree notation was formalized...

(still a brilliant read, though.)