Currently Being ModeratedJun 19, 2013 12:23 PM (in response to chac96)
Open System Preferences > Flash Player then select the Advanced tab.
Click Delete All under Browsing Data and Settings
Back to Safari. Press Command + Option + E to empty the Safari cache.
Quit and relaunch Safari, try a video.
If that doesn't help, you may have the YouTube HTML5 beta enabled.
Opt out here > YouTube
Currently Being ModeratedJun 19, 2013 6:09 PM (in response to chac96)
Currently Being ModeratedJun 20, 2013 10:33 AM (in response to Carolyn Samit)
For me, the HTML 5 beta was the culprit. Thanks!