Chromium派生ブラウザでYoutubeの再生に支障が出ていたが
OperaがYoutubeに報告、暫定対策も取り素早い動きをした
他の派生ブラウザが何もできずにいたのに、Operaはレベルが違うと鳥肌が立った

http://blogs.opera.com/desktop/2013/12/youtube-is-back/
Since yesterday, we’ve gotten a number of incoming reports that selected YouTube videos no longer play in Opera.
We’ve investigated this further and it seems to be a recently introduced codec selection issue on YouTube’s side.
We’ve contacted YouTube about this: their friendly engineers are on the case, and will release a fix for the problem soon.

In the meantime, we’ve released a browser.js patch that works around the issue by forcing YouTube to always use the Flash player for playing videos.
You should get the file 2-3 minutes after restarting Opera. We will remove this patch once YouTube’s fix is released.

Let us know if this doesn’t work for you.