The original idea for The Tenth Doctor's final story would have seen the TARDIS materialise on board a spacecraft carrying a family of aliens. The Doctor would sacrifice his life to prevent a radiation leak and save these ordinary, unimportant beings. Although Russell T. Davies liked the notion of the Doctor's regeneration taking place in such unremarkable circumstances - as opposed to the blockbuster events that had characterised each of his season finales - he was already concerned that viewers would consider this to be too great an anticlimax.
Donna was originally supposed to appear only in the epilogue for The End of Time: Part Two (2010), giving The Doctor a chance to see that she was enjoying a good life despite losing all memory of her time with him.
This is the first time the villain is credited in the opening credits.
Russell T. Davies originally wanted to have The Doctor and The Master swap bodies. However, Davies wasn't keen to spend time during David Tennant's final story with the actor playing anybody other than the Doctor, and was also mindful that he had used a similar notion in "New Earth".