I've submitted 3 videos to Youtube today. Last time I submitted videos a week ago I guess. I've noticed that in 2 of those cases today Youtube didn't take the middle frame for a thumbnail. I don't know if Youtube is getting smarter or just tries to add some randomness to make it harder to be manipulative with thumbnails.
Picking up good thumbnails has made a huge difference to me before. 1000% increase in views in some cases.