YouTube's cleaner look is part of a bigger plan to simplify the site based on user feedback and testing, as well as to keep people from ever leaving.
YouTube UI designer Julian Frumar explained that the site was simply not working like it should with the addition of extra features over the years. There were too many things on the screen that were slowing down page views and overwhelming people.
The redesign goes beyond the people watching, and has been tweaked for content owners as well. The company hopes this will spur more subscriptions, and give users a better sense of who made it.