The people over at YouTube are trying out a new UI. At the moment you have to opt in to see it - which you can do by clicking this link and activating it. Don't worry, you can always go back if you don't like it. However, so far I don't see much to not like.
The dark background is preferable to white for watching videos against. The resizing controls work much better and the UI as a whole feels more polished and responsive. There is less clutter, it's cleaner and it just feels much more slick.
Check it out and use the feedback tab to let them know what you think. Chances are, this could be YouTube soon, so might as well get in early and have a say in the matter.