I keep flash disabled in Linux, never accept cookies from Youtube (therefore I've not subscribed to their HTML5 channel or whatever they name it now) and so far I've had no issues with videos not playing on Youtube. As for embedded videos on other websites, it's a different story: even when the exact same video is also available on youtube in HTML5 format, most websites won't play the video other than with flash -since I could not care less, I simply ignore those videos.
mozillaZine is an independent Mozilla community and advocacy site. We're not affiliated or endorsed by the Mozilla Corporation but we love them just the same.