The new design is very much inspired by the one Google+ unveiled a few months back, down to the annoying amounts of whitespace used sometimes, made more visible by the left-centered layout.
Still, overall it's an improvement. YouTube is testing the new design with some users, but you can try it on for yourself, as always, by editing your YouTube cookies.
In Firefox, all you have to do is hit Ctrl+Shift+K, in Chrome it's Ctrl+Shift+J. In Opera, you can hit Ctrl+Shift+I and select the Console tab in Dragonfly.
You can find a more detailed explanation on how to do this for Firefox, Chrome, Safari and IE here.
First, go to YouTube.com. There, open the console as explained above. Then, paste this piece of code, courtesy of OmgChrome.com: