YouTube shows a small toolbar at the bottom of the page that lets you manage the videos from a playlist, more like the controls from a media player. The toolbar has options for pausing and muting the active video, buttons for playing the next video from the playlist and for automatically playing videos. All of these options were already available in the sidebar, but the new toolbar makes them more visible and it follows you in any YouTube page.
The excellent music sites The Hype Machine and thesixtyone have similar toolbars, but I'm not sure if the concept works well for videos. I can only see the new feature in Firefox, so I assume this is yet another experimental feature.