Sharing what I've learnt from a few months of making Youtube Thumbnails
Everything has to be clean - Corners, edges of images, everything, otherwise the thumbnail will look cheap and unappealing.
The simpler the better - Attractive blurred colourful background, with subject and text popping out at you. Avoid white and colours that will blend into the Youtube backdrop
Every subject needs to have a glow/shadow around it. This gives it that professional look. Black silhouette works best, but white and other bright colours sometimes are the best choice
Keep all your thumbnails similar so you build a brand identity, your viewers should recognize your thumbnail style