The shape of the crack in the wall and universe, in this and all the stories where it appears, is in the same shape as the outline of the mountains behind the town of Christmas, as seen in the wide shots of the horizon of the town.
Steven Moffat's original plan for season eight saw the Doctor set entirely on Trenzalore. Matt Smith's departure meant that he compressed the idea into one special.
The Doctor stops one of the Weeping Angels by placing a mirror in front of it. Written on the mirror are the words "With Love from The Doctor". This was the same message the 10th Doctor left for Sally Sparrow in Blink (2007), the introduction of the Weeping Angels.
Matt Smith underwent knee surgery shortly before filming began, and so Steven Moffat had the Weeping Angels injure The Doctor's leg, resulting in it eventually turning to stone and being replaced with a prosthesis. (References to this wooden leg would ultimately be deleted in editing, when it was decided that because the artificial limb was never shown on screen, the notion came across as superfluous.)