Leanback is perfectly suited for watching youtube videos on televisions that are internet enabled. When the HD version of a video is available leanback will play that. The controls are very easy too and you can use your keyboard to control it. In fact you can control almost all the functions with just the arrow keys and the Enter key, except when you are entering a search term.
The videos fill up the entire browser area and shows the maximum viewable area which is just right for watching on a TV.