[NCC1701DEngage]

It is pretty obvious as a creator. The analytics available encourage experimentation. You find that mentioning subscribing or making a more visually engaging thumbnail will significantly boost important metrics like CTR. CTR is a metric almost entirely dependent on thumbnail image, although title and description play small roles, and CTR is an incredibly important metric for getting the content served by the algorithm.

And it isn't entirely true that including a face mugging for the camera is always the most visually engaging (although representations of people are very attention grabbing). A person in the image is a character for a story, the whole "worth a thousand words" is true in that a whole narrative can be compressed into a single image and viewed with a glance. A person or other sentient being is a character for that story (I'm sure cat thumbnails do well too).

Just try to think of an interesting narrative involving exclusively inanimate objects. Kinda hard; the whole "animate" thing seems to be necessary to give a before, middle, and after to events (ok, maybe collisions, explosives, and rockets might work, they'd probably do well as thumbnails too). A thumb with Linus looking surprised or disappointed or puzzled gives a short and incomplete narrative about the object he is looking at and how it made him feel those emotions. Part of this is that watching the video will give you a more complete narrative.

[joegahona]

"Please take a moment to subscribe" and "smash that bell icon" are the video version of "Subscribe to my crappy newsletter" popups. And before that, in the world of print magazines, it was multiple subscription cards that fell into your lap -- even if you were already a subscriber. The reason is an unsatisfying one: They work, and it's difficult to measure penalty metrics that show they're causing more harm than good.

I would think a human face would be the most effective thumbnail, and that there are psychological reasons for this. When I worked in print magazines, there were metrics thrown around a lot for how well cover subjects did in terms of newsstand copies sold: Animals > inanimate objects; Humans > animals; color > b&w; photos > illustrations; eye contact with the camera > looking away; females > males.

Apps are also getting into the fun, and tons are now including screaming faces[1] as their icon, no matter how related to the gameplay. Can't wait for a productivity app trying to pull off this icon...

[1] https://i.redd.it/2t190ls2j2571.png