I can only really speak to fashion history. I can however, say that downton abbey is marvelously well-researched in that respect. Lady Sybil's version of a controversial Paul Poiret outfit, for example. I've seen several displays of the work at a fashion museum here in LA, and every detail down to their jewelry has intention and thought. For example, what pieces would be era and cost appropriate for a woman getting her first pieces of jewelry coming of age in the early 20s, vs the late teens, etc. The fashion is ALWAYS incredibly well researched, with the dowager countess wearing hair and clothing that her generation would have been more comfortable wearing. The detail for every single character, to me, is quite mind-blowing. When you see the costumes and pieces first hand it's even more remarkable.

Slightly off-topic, but I've seen the research binders first-hand for Mad Men and quite a few other productions. Every single episode has binders of catalog and newspaper references, ads, images. Hundreds of hours of research go into many of the more detailed productions these days, you would be amazed at the level of detail and primary sources they reference. It's a bit of a dying art in the explosion of internet images and smaller budgets, but there are exceptions. I believe downton abbey is one of them.

I can only speculate that much research has gone into virtually every other aspect of the show. There are a few BBC documentaries and articles on the topic available, even on netflix, about the estate that inspired the series and its occupants. They speak of much of the research there. Of COURSE there are significant liberties taken with plot, I presume, because it's the entertainment industry. However, I've spent the last 23 years of my life loving and learning 20th century costume and fashion history, and I would go weak-kneed to meet the team behind this genius.

Downton Abbey irritates me immensely because its central theme is the interplay between the family and the servants. It just gets its catastrophically wrong.

For a start, how many servants are there in DA? A dozen? Ok, look, a house that size would have had a staff of hundreds of servants. For example when the Duchess of Devonshire died, some 600 of Chatsworth's staff attended. And Buckingham Palace has a staff of 800!. Even comparatively small weakend retreats like Polesdon Lacey would have a staff numbering in the hundreds, let alone a Lordly seat like DA...

The interplay between staff and the family is ludicrously familiar as is the concern the family show for the welfare of their staff. Mrs Greville once sacked a Chauffeur for picking her up in the Mercedes instead of the Panhard.

Life in these houses was organised so that the staff and the family had as little contact as possible. If the Madam of the house passed a maid on the stairs, the maid would avert her eyes and the Madam would ignore the maid.

Houses like Knole actually have separate sets of stairs leading to the various room, the doors disguised, so that servants could come and go without inconveniencing anyone with their presence.

At Lanhydrock

the servants’ areas were located well away from the living quarters of the family to ensure that the family would not accidentally bump into any of their staff.

There is a network of bells that stretch around the house, which the family pressed to call a servant if they needed one.

It wasn't just the family and the servants who were kept apart. Male and female staff were kept completely separate, even to the extent of having different stairs and doorways to prevent them coming into contact with each other.

The only time everyone met up was at mealtimes in the Servants’ Hall, when everyone would sit together in a strict order of hierarchy.

The butler and the housekeeper would sit at either end of the table and the other staff were seated in order of status. Meals were eaten in silence.

The staff were effectively non-people

Generally Staff got half a day off per week. They got up at dawn and worked late into the night. They were of no more import to the family than any other easily replaceable commodity.

The notion that the staff would be on such familiar terms with the family is fatuous.

The show seems to take a lot of trouble getting fashion and technology right, and I notice they're even pretty good with typefaces (though a rel Yorkshire village would have hand-lettered signs, not ones from a laser printer). More troublesome is anachronistic language. I particularly notice intrusions from modern business-speak, things like Carson saying "the bottom line is . . . ".

I can't speak for the rest of it, but I remember one particular, maybe a rather small example of anachronism sticking out to me from the first season about that handsome Turkish diplomat I'm sure you remember: his name, 'Kemal Pamuk'. Note that it's the year 1913, so Ottoman Empire is still standing, and muslim populations of the Ottoman Empire (unlike its Jewish and Christian minorities) do not have surnames yet—they only held titles like 'Pasha' or 'Bey'. The Surname Law was passed 21 years later, after the modern Turkish Republic was founded. "Pamuk" is not a title (means 'cotton') and is used strictly as a surname, so the young diplomat couldn't have possibly been named Kemal Pamuk.