Steven Moffat hasn’t always been successful as a show-runner of Doctor Who. I’ve enjoyed his work more than some others have, but it’s obvious he’s sometimes struggled with keeping it all together. I think he’s been done in by modern television’s insistence on “seasonal arcs,” which is excellent for some shows, but it’s something far too many programs that are ill-suited to it have forced themselves to adopt rather than just stick with standalone episodes and progressing character arcs. (This need for ever-escalating seasonal arcs pretty much killed Burn Notice in its final two seasons, and severely damaged later seasons of Buffy the Vampire Slayer, just to give two examples.)
Part of it is Moffat’s own ambition—he’s not satisfied with just dropping a phrase (“bad wolf”) or a reference (Torchwood, Prime Minister Saxon) into every episode, he needs there to be a big Rubik’s cube of stuff that has to come together with each color on the right side at the end. The problem being, of course, that he doesn’t always succeed in that. (The whole impossible astronaut thing, for example, didn’t quite come together as well as it should have.) I think the arc in this most recent season worked well in part because it was scaled back somewhat to simply the mystery of one character, as well as paying tribute to the show’s history (in its 50th anniversary year) by bringing back an old bad guy in the Great Intelligence.