HTML5 (WebM) just ruined it. Rated 3 out of 5 stars
Even if some fixes work for you, that doesn't mean it's gonna work for everybody. Clearing LSOs cache did 'nothing'. From what I found after I searched around the web, I'm thinking it has a problem with HTML5. People said that some videos start to use HTML5 now.
Edit: Ok, I think it's clear now. Some people don't have any problems because they are not using a YouTube account. We are apparently in a 'trial' of HTML5 video player. Even Adblock Plus' little tab doesn't work either.
***You can disable YouTube HTML5 at (youtube.com/html5)