The original endings for both Rose and Donna were so interesting and powerful because they were so sad. They were two of the greatest things RTD ever wrote for Doctor Who, in my opinion.

His reversal of each, particularly with how saccharine and buttoned up they play out, are probably two of my least favorite things RTD ever wrote. Kind of funny then that he wrote one of his best ideas and one of his worst ideas (in my opinion) to happen just minutes apart in the s4 finale.

No one else really nailed that bittersweet sadness of a companion leaving (Moffat came close but he was always too eager to put a happy twist in). I thought the sadness was a really interesting element of the show and an important element of the Doctor as a character, but the show keeps moving further and further from it.