Gotta get rid of the "&feature=youtu.be" tag at the end for it to automatically embed...and because you already posted it, you have to remove the bbcode in BBCode mode (or just remove it entirely and re-paste the link without the aforementioned tag) because the forums automatically hotlink it with the original link that you posted.
See above.
(e): I might as well take this opportunity to complain about the BBCode mode being broken again, because yes, it's still broken, and yes, the default non-BBCode mode is terrible, and so the current state of affairs is also terrible. It sure would be nice if it were fixed.
Always a good idea to scrub the POST/GET stuff off URLs anyway. Amazing the amount of junk in there sometimes.