I am no UI expert so I will not try to evaluate the old vs new UI. However, I want to say that I really like the way Github has looked and functioned. It feel lightweight and it feels like someone just built it up from basic html. For example, you can right click the "go to file" button and open it in a new tab and it works like any standard link. Compare that to any button on facebook or youtube or gmail which are probably not even real html buttons, you can't expect them to properly open in a new tab.
