“We’re expanding the width of the page to 960 pixels to better reflect the quality of the videos you create and the screens that you use to watch them. This new, wider player is in a widescreen aspect ratio which we hope will provide you with a cleaner, more powerful viewing experience. And don’t worry, your 4:3 aspect ratio videos will play just fine in this new player. ” —YouTube Blog
“The cachet of widescreen should do wonders to bolster YouTube’s already high reputation for quality. Suddenly, watching a cat flush a toilet in 4:3 doesn’t seem right.” —Ken Broomfield, After Effects artist and developer of the iRider browser.
On the other hand, it does look a whole lot better fullscreen than it used to.