As was pointed out as recently as Disharmony (2001), soulless vampires are not truly humans who have been transformed into vampires, but demons who inhabit the bodies of dead humans to become vampires, their personas shaped by the humans' memories. Prior to regaining his soul, Angel, originally named Liam, was such a being, a demon who, in combination with Angel's original personality, became Angel's evil persona Angelus; only the restoration of Angel's human soul (e.g. the original Liam whom the demon supplanted) suppressed the demon/Angelus. Angel's demonic form in this episode, which he inadvertently assumes when displaying his vampiric nature, is thus presumably the demon who originally possessed Liam's dead body, looking as it would without the human influence that originally caused it to become Angelus.
The title is from Lewis Carroll's Through the Looking Glass. It is the sequel to Alice In Wonderland. Through the Looking Glass is most well known for The Jabberwocky. Another ode to Lewis Carrol is Cordelia saying, "Off with their heads!" said by the Queen of Hearts in Alice in Wonderland who later blends with the Red Queen in Through the Looking Glass. Alice also goes through the mirror to enter another world just as the main characters have entered into another world.
When Angel looks in to the mirror, he attempts to flatten his hair. Ironically enough, the general poofiness of his hair is something Spike mocks him for many times.
This is the only episode of series not to have any scenes in the Earth dimension.
When Angel is telling stories to the Pyleans, he refers to the events of the first season finale, To Shanshu in L.A. (2000), in which Angel cuts off Lindsey McDonald's hand in order to save the Scrolls of Aberjian. In the story, he refers to Lindsey as a "lawyer-beast." There is also a reference to the events of I Fall to Pieces (1999); Landok asks Angel to tell the story of the "sorcerer who could remove his limbs and reassemble at will."