As seen in the attached screenshot YouTube's new playlist bar at the bottom of the page doesn't function very much. Instead of video thumbnails it appears to try to load a regular Youtube page. All controls on it are unresponsive. In Safari 5.0.1 the video thumbnails load but there's major flickering when they're hovered over. In Chrome the bar seems to function fully, and it's there that I see that it's meant to load in a minimized state, with just the very top grey bar visible until the cursor is hovered near the bottom of the page and it then maximizes. When the cursor is moved away it drops to a minimized state again. In Webkit (r64341) it does none of these things and since the controls don't work, it can't be minimized at all.
Created attachment 63149 [details]
Screenshot of issue
At least one of the issues you mention is known as bug 41622. If there are other issues you see with recent nightly builds, please file separate bugs for those.
*** This bug has been marked as a duplicate of bug 41622 ***