Tear Jerker / Thief of Time

• The revisiting of the idea that Death is possibly the most lonely being in all existence, as realised by Susan in Soul Music, and underlined in Hogfather... but with the addition that not only is Susan the only person he really has, but vice versa too: Susan's slowly and inexorably drifting out of normal mortal reality and she can't do anything to stop it.
• The imagery of Time herself walking through her glass palace, forever unable to interact with her son (or, technically, sons, depending on whether or not one counts Lobsang and Jeremy as two separate people), is both desperately sad and profoundly haunting.
• The dignified suicide of Unity, the Auditor who became human, found that she loved it, but also that it made her incurably insane to the point that she just couldn't go on and chose to end her new life on her own terms. And then after dying, she discovers that she has also developed a soul...