You can expect greater speed, a much improved UI and a mobile site that closely mimics the main site in terms of the overall experience. The new site might make native YouTube applications on various smartphones seem outdated.
“As the world continues to go mobile, we think this is a great improvement for users who want a more consistent YouTube across many devices, no matter where they are. We're launching in English only today, but will be rolling it out in other languages in the coming months,” YouTube said on its blog.
You can watch the demo video below. Or better yet, direct your phone's browser to m.youtube.com and experience the changes first hand.