The point being:
I merely copy/pasted that video's URL from the YouTube address bar. I didn't use any square brackets or any of yer fancy book larnin'.
I find that if you just post a URL on its own, however, the reply box doesn't register that you've entered anything in it - you get an Error message.
So you have to accompany the video URL with a little typed message of your own, either just above or below it. Above is better.