New rules to ensure post quality. See this thread for details.
See, now the video you posted isn't working.
This may seem like a silly question, but did you use the whole URL?
I'm a member of another forum where the YouTube tag works with the portion of the URL that appears after the equal sign (so, for example, if the URL were //www.youtube.com/watch?v=hMfq_J4h9j8 you would wrap the YouTube tag around just "hMfq_J4h9j8" and omit the rest), but on this site it requires the whole thing.
Also, I believe some YouTube uploaders somehow request that videos not be embeddable, so could that be the issue here?