If you click on your link, crav, you will see in the browser that there is more to the URL. You posted
http://youtu.be/TrL_8ztB6-k
But the URL is actually
http://www.youtube.com/watch?v=TrL_8ztB6-k
If you copy in the paste the second URL into a post, and then add http:// to the beginning of the pasted URL (if it isn't already there), you will see that it will embed automatically.