Conventions of Period Drama Series (Downton Abbey)

Having not watched a large variety of period drama series, it is difficult to distinguish all of the common conventions, however I compared my first impressions of the first episode to research that I did on period dramas in general, some points which I agreed with and some which I didn't find (yet) in Downton.

First Impressions

After watching the first episode and doing some research I concluded the following three points are above all, the main conventions of period drama tv:

• Heavily reliant upon mise-en-scene. This entails accurate period clothing and set design. 
• Based upon/or simply includes a distinguished historical events/characters/perhaps even narrative. This could could also mean the series is set during a well known time period or around the time of a famous event (for example- Victorian Era, WW1, Tudor etc...). The BBC production called "Lost in Austen" is an example exploring a famous period narrative of Jane Austen- Pride & Prejudice. 
• Addresses love, family and relationship problems- what propels the plot and keeps the series going (use of intricate story lines) 

Conventions obtained from research: 

• High production values (of course this will be due to the accurate portrayal of a certain period of time which requires a lot of thought to be put into costume + set design. This also entails the use of known period actors such as Maggie Smith and Hugh Bonneville which will immediately draw attention as these are respected and famous actors expected of a satisfying performance. 
• According to one source they "criticize the dominant ideology of the time" which I'm not sure I've come across yet in Downton Abbey. 
• Modern day ideas/themes explored strongly through the period of the time, to still relate to the audience but remain in-keeping of the reality of the time period being explored. Apparently sexuality is one theme that is quite commonly explored through period drama, this was already evident within the first episode of Downton.