YouTube has already changed many times its interface, but now we have a general picture of how the site will look like when the source of inspiration for the style will be Material Design.
It is already possible to launch a new look, but it is connected with a number of limitations. You can check how YouTube will look in Material Design only by using Google Chrome and only when there was no users logged in.
Basically, all elements of the interface have been changed. Both the main and side menu recall at this moment more extensive Android application than the current form of the service. Also, side channels have been changed: the elements adapt to the color of the main image.