Despite his considerable achievements, Moffat's tenure has not been without problems. His well documented love of exploiting time travel mechanics for plot purposes - lovingly handwaved by the Doctor as "wibbly wobbly, timey wimey...stuff" - has often led to overarching plots that feel ultimately pointless, giant puzzles for the Doctor to solve and then put everything back how it was. The Doctor's sole understanding of those "timey wimey" rules means he's almost always two steps ahead of everyone else, including the viewer, which often felt like a cheat. It also led to the Doctor's occasional elevation to something akin to a god, capable of such limitless power and knowledge that the stories felt robbed of genuine peril.
Perhaps Moffat's greatest sin has involved the Doctor's allies and companions. With the exception of season 10's Bill - who felt like something of an apology for the previous missteps - every companion in Moffat's tenure was, on some level, a human puzzle box, less a fully formed character and more a mystery for the Doctor to crack.
Two characters in particular suffered tremendously from this effect. Clara Oswald was introduced as a far future survivor of a spaceship crash...then a barmaid in Victorian England...then a nanny in the present day. The question of Clara's existence was allowed to linger for a full season, before it was finally revealed she was split over time when she jumped into the Doctor's timeline to save him from death, a plot point that was both maddeningly vague and reduced Clara's entire purpose in life to saving the Doctor. Later seasons would attempt to make Clara a more fully realized character, but she could never completely shake that insultingly lame initial arc.
Easily the most divisive character of the Moffat era has been River Song, played by Alex Kingston. River first appeared in the Davies era two-parter "Silence in the Library/Forest of the Dead" - penned by Moffat - where the Doctor first met her, but where she already intimately knew the Doctor. Despite her death in that story, she would continue to feature in Moffat's era, as her life essentially played out in reverse to the Doctor's. What was once an incredibly enticing mystery and ingenious use of time travel eventually curdled, as River was revealed to be obsessed with the Doctor in decidedly unflattering ways, yet another female character who seemingly existed for no other purpose than to impress and love the Doctor. River is in many ways a microcosm of both Moffat's greatest strengths and most frustrating weaknesses as a storyteller.
Even acknowledging Moffat's narrative blindspots and the growing feeling that he's said all he has to say about the character and his world, his departure is a bittersweet moment. Other than composer Murray Gold, Moffat is the last major component remaining from the revival series' earliest days, and his exit feels like the end of an era in an even more profound way than Davies' curtain call in 2010. Chris Chibnall has written a handful of Doctor Who episodes over the years, but he's made his name as a writer outside of the series, most notably in Broadchurch, a show with a decidedly different tone and aim than Doctor Who - even if it starred both past and future Doctors in David Tennant and Jodie Whittaker. Doctor Who's future has never felt more uncertain, which is in some ways exciting after nearly a decade of Moffat's tenure.
Whatever the future holds for Doctor Who, it's going to stand on a foundation built by Steven Moffat. That foundation isn't perfect, but it's been a creatively audacious, narratively rewarding era for the show that honored the past while boldly pushing the show into uncharted territory. In both thrilling and frustrating ways, Doctor Who has never felt more quintessentially Doctor Who than under Steven Moffat's tenure, which is perhaps the greatest compliment you could pay a loving steward of the TARDIS.