Here is my opinion about the layout and why it works.
First, we as humans tend to read from left-to-right/right-to-left. We all know that.
Long line lengths are bad for reading, but that doesn't necessarily apply to youtube, unless you enjoy reading the comments while listening to the video.
Even if we take into account our reading direction, that doesn't mean that text should start either on the far left or right. It makes the reader feel as if they've been backed into a reading corner. We read left/right but we scan things up and down along the center.
We humans tend to want symmetry and putting content in the middle is where we'll feel most comfortable looking at content. When we look at someone we don't look at their left or right ears to concentrate on them. At least I don't because I would feel odd if I did. That's why I think the layout works. The content, or what we want to concentrate on, is in the middle.
Reddit could definitely improve it's layout and would benefit from a layout where it's main news content is centered on the page and doesn't suffer from long lines. They could make their menus larger and easier to click on up at the top, and they would have more room for additional content on the sides while leaving their main content in the middle, where we tend to want to look anyway.
Youtube - what feels better, looking at a centered video or looking at a video at the extreme left of the screen? Navigation on the left or right is fine. Navigation is secondary to content. Content is the prime subject. Imagine if you placed your tv on the for left or right corners of your wall? You might have to because of the room constraints but it feels better in the center.
Facebook - content is in the center. The content is the news feed and what our friends are saying. All the stuff on the side is secondary. Who wants their news feed on the extreme left or right? What would be in the middle? menus? navigation? No, the content. And the content is the information about our friends and family members.
Stackexchange - The point is to see what kinds of questions people are asking. The questons are the content and are in the center. All the stuff on the side and the top just enables you to get to questions that are relevant to you. Again, the main content is in the middle.
That's my opinion. Content is king and it should take center stage....not be off on the left of the stage or the far right where you have to look for it.