It's not buggy, that's how OS X behaves without hardware acceleration, it's all related to the lack of graphic hardware acceleration in OS X guests, and that hasn't changed since it depends on Apple giving the necessary access to the required APIs, which hasn't.
As for Youtube, or any Flash content for that matter, you may try to disable hardware acceleration via the flash settings (left click on the flash content) and see if it gets any better.
Last edited: Feb 14, 2013