My initial problem with parentheses wasn't the stacking of them -- that's inevitable even in elementary arithmetic -- but with the fact that x is not the same as (x), which doesn't happen in math and doesn't get emphasised enough in Lisp intros. Once I got that, everything was easy.

Edit: To answer to the responses to this comment, it's not that it's conceptually hard, it's just that the parenthesis symbol in its usual incarnation does not work that way, which leads to confusion.