8. Casting Nepotism
It began when Sherlock writer and co-creator Mark Gatiss cast himself as Mycroft, only a bit part in the early seasons, which he played well as a fusty and distant older brother of Sherlock. However, as the show went on this nepotism only got worse.
"Sherlock largely runs on nepotism," confessed Steven Moffat. "I'm married to the producer, one of the execs is my mother-in-law, Mark (Gatiss) and I are old, old friends, Martin (Freeman) is working with his partner Amanda Abbington. So generally speaking we just don't like to go outside our tiny circle very much."
Instead of focusing on the series, Moffat and Gatiss seemed more obsessed with constantly nudging us at the genius of casting Benedict Cumberbatch's real life parents as Sherlock's parents or casting Martin Freeman's partner as John's love interest. These decisions seemed to be nothing more than a publicity stunt which also managed to deny any other talented actors the opportunity to star in the show.
Perhaps most worryingly of all this nepotism ultimately resulted in an echo chamber and clique-like mentality within this 'tiny circle' who were unable to see the shortcomings of the later Sherlock seasons. It is no coincidence that the beginning of season three, during this peak complacent nepotism, saw a rapid deterioration in the quality of writing.