Downton Abbey has aired its Grand Finale in America, which means the show now feels as though it's truly come to an end. In the lead-up to the big event, I've been playing all the show's previous seasons as background noise while going about my daily tasks, just to get a big-picture sense of what the show was and where it ended up going.
Back when it first started, I recall Michelle Dockery saying in an interview that the show's appeal lay in the popularity of literary period dramas (such as those based on Jane Austen and Charles Dickens) but with the significant bonus of nobody knowing where the storylines would go. Everyone knows the plot of Great Expectations and Pride and Prejudice, but Downton Abbey's originality lent the show a genuine sense of suspense in the progression of its relationships, scandals and character arcs.
It's as good a reason as any to explain its popularity, though having watched through all six seasons (and the Christmas Specials) it's obvious Julian Fellowes was flying from the seat of his pants much of the time. Let's say, from season two onwards.