C. The Writing.
And this is what we really get down to. The writing. Early nuWho had its share of clunkers. The last two Doctors had some decent episodes. But when you sit down and make a list of the stone-cold classic nuWho episodes ... what episodes are you coming up with?
Dalek? Silence in the Library? Midnight? The Girl in the Fireplace? Blink? Turn Left? Eleventh Hour? Vincent and the Doctor? The Doctor Dances?
When it comes to the last ten years, how many episodes would crack the top 50? Top 75? Probably the Heaven Sent two-parter. Perhaps Listen? From the Whittaker era .... maybe Demons of Punjab?
We are on a ten-year run of diminishing returns when it comes to the writing. And this is evident in the finale- you shouldn't need that much fan service to send off the Doctor. There should have been enough pathos and characterizations done the entire time. The episode where the Doctor leaves should be about the Doctor in such a way that it is unmistakably about this Doctor and this characterization.
And here, the show's writers did a disservice by having this all happen to the Doctor. Even ignoring the plot issues, how did this specifically apply to the characterizations of this Doctor so far ... I mean, she was like the Dom in the Fast and the Furious, in that she was about family, and she liked gadgets, but when you get right down to it ... The Master out-thought her, she was forced to regenerate not of her own accord even when she wanted to continue being the Doctor, and they ended with a poorly developed nod toward the late-developed relationship with Yaz.
Cool cool, right? So what does all of this mean for the show going forward?
Well, I am super excited for Ncuti Gatwa, but also ... not? Let me explain. In the history of show, the switch from Ecclestone to Tennant was difficult. But Tennant nailed it to the extent that, for many people, he is the Doctor. And then there was the switch to the then-unknown Matt Smith. Of course, his first episode was a classic, and he was also a great Doctor.
The last two Doctors (Capaldi and Whittaker) were both incredibly accomplished actors and I was super excited for each of them, but both of them were let down by the writing. And I think that's the lesson- I think Gatwa could be amazing (he is great in Sex Education), but all that matters is the quality of the writing. I think that RTD understands a lot about the Doctor, and I am glad to see him back ... but I hope be brings fresh energy (and new writers) to the show. His run at the end of the Tennant era was getting tired, and the issues with the show now are not necessarily what he is best at solving. While I am glad to see Tennant reprise his role for a while, and Catherine Tate (of course), the show doesn't need more spectacle.
It needs a return to basics. IMO.