Apparently the problems with YouTube are not just with Macs. If you visit
http://www.google.com/support/forum/p/youtube/thread?tid=5b6f8b8b8014a6d6&hl=en& start=120
you will find complaints from Mac, Windows and Linux users. Similarly, users had various browsers which were malfunctioning; IE, Chrome, Firefox, Safari.
Someone called "YouTube Diego", purporting to be a YouTube employee, posted the following message on 2/17/2011, requesting feedback from users:
"+Some of you are having issues watching YouTube Videos. If you can hear audio, but are not seeing the video, you've come to the right place. Please respond below with the following information...the more info you provide us the sooner we can help you:+
+1) What do you see: (a black box, white box, a green line, weird colors -- please be specific)+
+2) Your Operating System (Mac OSX, Windows 7, Windows XP, etc)+
+3) Internet Browser and version (Safari, Chome 7, Internet Explorer 7, Opera, etc)+
+4) Video Card if you know it+
+5) Adobe Flash version if you know it+
+As a temporary workaround, you can try one, or both, of the following:+
+1) Right click on the player > click Settings > Disable Hardware Acceleration > Refresh your Internet Browser+
+2) Try closing your browser and re-load the video.+
+Please let us know if you have any further questions. We really appreciate your detailed feedback. Thanks for your patience+."
"YouTube Diego" refers to a problem with hearing audio but seeing no video. I think that my problem with YouTube is like most of the folks here; no controls shown to start the video, so therefore no video or audio. In the listing of feedback to YouTube's site, I saw other people who reported the same symptoms as ours.
Pete