The "guide" sidebar is displayed by default only if there's enough room. If there isn't, the sidebar is hidden and you need to click "guide" to see your playlists, subscriptions, video history and more.
The layout still uses cards and it's center-aligned.
Here's how you can enable the experimental interface. If you use Chrome, Firefox, Opera, Safari or Internet Explorer 8+:
1. open youtube.com in a new tab
2. load your browser's developer console:
* Chrome or Opera 15+ - press Ctrl+Shift+J for Windows/Linux/ChromeOS or Command-Option-J for Mac
* Firefox - press Ctrl+Shift+K for Windows/Linux or Command-Option-K for Mac
* Internet Explorer 8+ - press F12 and select the "Console" tab
* Safari 6 - if you haven't enabled the Develop menu, open Preferences from the Safari menu, go to the Advanced tab and check "Show Develop menu in menu bar". Close Preferences and then press Command-Option-C to show the console.
* Opera 12 - press Ctrl+Shift+I for Windows/Linux or Command-Option-I for Mac, then click "Console".
3. paste the following code which changes a YouTube cookie:
document.cookie="VISITOR_INFO1_LIVE=rkZGzYxfbLQ; path=/; domain=.youtube.com";window.location.reload();4. press Enter and close the console.
To go back to the regular interface, use the same instructions, but replace the code from step 3 with this one:
document.cookie="VISITOR_INFO1_LIVE=; path=/; domain=.youtube.com";window.location.reload();{ Thanks, Florian K. }