For example, here's something like how a Schemer might implement filter (although this seems long to me...am I missing something?):
(defun filter (predicate list)
(if list
(let ((head (car list))
(tail (cdr list)))
(if (funcall predicate head)
(cons head (filter predicate tail))
(filter predicate tail)))
'()))
(defun filter (predicate list)
(loop for elem in list
if (funcall predicate elem)
collect elem))
So yeah it's worth it!