From a guest post by Josh Farmer on ShawnBlanc.net:

Downton Abbey uses shaky handheld cinematography when the subject is the servants, but steady and composed shots when the wealthy figures take screen time.

I hadn't picked up on this simple detail despite being a big fan of Downton Abbey. It's a detail that seems so obvious once you hear it, but obviously it's done subtle enough that they don't slap you in the face with it.

Plenty more fun details in the full article.