So, when using the gnash plugin, youtube videos don't start until the user clicks outside of the video, which is highly non-obvious. It seems only problematic with gnash, not adobe's flash. See debian bug report for more detail: http://bugs.debian.org/644115
Mass closing plug-in bugs, as plug-in support has been removed from WebKit. Please comment and/or reopen if this still affects WebKit in some way.