I believe it is the humiliation aspect of standing there knowing he is about to inflict this pain/pleasure on me so it is physical as well as mental. As he paces behind me back and forth back and forth....I can hear his footsteps, know where he is, but cant guess exactly when the next "thwack" will occur.
As far as other places...I love the crop on the chest and kitty as well...especially on the nipples...though as mentioned already it is a delicate line!!