Thumbnail design.

A thumbnail and a title are the only two things you get to win the click, so I treat a thumbnail like a tiny piece of storytelling.

My taste is clear and bold over cluttered: one focal point, a face or a moment that tells you what the video is about, readable at phone size, with just enough intrigue that you want the answer. I come from editing, so I always think about the video behind it, and make sure the thumbnail promises something the video actually delivers.