Blackheifer and broken Youtube tags. Name a more iconic duo.
Wait, how is it broken? What needs fixed?
There you go, fixed it for you

When the correct link is popped in the tags it appears smaller in preview rather than taking up a massive amount of screen, and you don't actually see the "video" tags appear on your post. I think you were missing the "watch?v=" bit of the hyperlink that the tags seem to need to process correctly. At least, that was the change I made to get it displaying properly.