1. The main characters' tastes in music would all tend towards 18th century classical music. Occasionally, characters would get into arguments over one person's preference for Mozart over Bach, which the other dismisses as "just noise".
2. Foolish characters or great wits would be referred to as "a regular Thomas Betterton/Elizabeth Barry"; a particularly wacky or comedic situation would be referred to as "like something Thomas Sheridan would have come up with."
3. At least one character would have an eccentric fondness for leyden jars or spinning jennys, building them in his spare time as a quirky character touch.
4. At least one character would have a desire to travel by paddleboat, horse, or railway carriage, feeling that cars and planes "lack romance".
5. Characters would frequently and casually analogize events in the present-day as being "somewhat akin to the Treaty of Westminster helping to cause the Seven Years War". Nobody would ever need the Treaty of Westminster explained to them. Nobody would ever confuse it with the Treaty of Westminster that ended the Third Anglo-Dutch War, either.
6. In a related issue, people would recognize the Prussian, Ottoman, and Austro-Hungarian flags on sight.
7. Everyone would exclusively quote Shakespeare. EXCLUSIVELY.
8. When not wearing their professional outfits, people would dress in slightly more utilitarian versions of waistcoats, ruffs, and powdered wigs. (The powdered wigs would be smaller, for example, to show the changing times and fashions.)
9. Sports fans would follow cricket, boxing or horse racing, and would occasionally express a longing to be able to travel back in time to the days when Lumpy Stevens beat John Small only for the ball to pass through the wicket without being disturbed. They would, on occasion, insist that the sport was better before they added the third stump to the wicket.
10. If anyone ever did reference a modern-day piece of pop culture, whether in the form of music, books, comedy, theatre, movies, or television, it would be only in reference to an actual celebrity in that particular field visiting them. The celebrity in question would never actually perform in their chosen field, but at least one character would always have been a fan.