Netflix shared a few other interesting notes about viewer habits, including how images are the dominant way users determine what they'll watch, and that images take up 82% of a user's focus while on the site. Amazingly, each film gets only 1.8 seconds of attention as users scroll past, so the image has to grab your attention in a hurry.
Additionally, Netflix discovered that strong emotions and faces drive viewership, and a single recognizable character in a show image outperforms an ensemble image of the whole cast. You'll notice new thumbnails for The Unbreakable Kimmy Schmidt focus on solo images of Elie Kemper or Tituss Burgess expressing big emotions. The same goes for the plethora of character promotions in advance of the second season of Daredevil.
The whole villain-love thing possibly explains the decision to give Jon Bernthal's The Punisher its own show, too. Netflix, however, doesn't mention whether or not they think Francis Underwood is a hero or a villain, and their answer might totally be skewing the data.