Rose becoming Bad Wolf was one hell of a deus ex machina, but it was a fantastic story, so why not? The resolution 'Doomsday', where the Doctor sends the Daleks and the Cybermen to hell used elements of the established story, so it made sense. The Series 3 finale, where the Doctor rises like Jesus to wipe the previous year of history was somewhat contentious. But the Doctor-Donna? Surely that's one deus ex machina too far? Russell T Davies is a master of the build-up; 'The Stolen Earth' united the Doctor's companions, and 'Journey's End' did a brilliant job of developing the Doctor's relationship with all of them, as well as his rivalry with Davros. It seemed like the Meta-crisis Doctor and Donna would save the world, but then they were also thwarted. It seemed like there was no hope, but then out of nowhere, Donna was given all of the Doctor's powers. Together, her and the Ten Clone then destroyed all the Daleks with the press of a button, and the Doctor was left claiming that he should have seen it all along: "The Doctor Donna: just like the Ood said!" What was an excellent set-up descended into the same old trick; the Daleks all died with a click of Davies's fingers, and Donna got her turn to be godlike too. Perhaps most upsettingly though, all that wonderful character development just vanished, as Donna completely forgot her life with the Doctor at the end of the episode.
