Nested Labels is just a cosmetic change that lets you create labels which are displayed hierarchically. If you enable this experiment and create a label like Mailing-Lists/Linux, you'll notice that Linux is displayed as a subfolder of Mailing-Lists. Unfortunately, all the other places that let you interact with labels show the label as Mailing-Lists/Linux.
"You can create complex hierarchies of labels if that's the way you like to organize your mail, and you can expand/collapse labels to save space. You'll always be able to tell whether a given label contains unread messages in its collapsed child labels by looking at whether it's bold or not," explains Google.
Message Sneak Peek shows a small preview of a conversation, so you can quickly read the first sentences without opening the conversation. Right-click on a conversation from the inbox or from another view and you should see the first unread message from that conversation. If you've read all messages from a conversation, Gmail shows the last message.
There's also a keyboard shortcut for showing the preview pane for the currently selected conversation: h. You can navigate to the previous/next conversation using k and j or dismiss the pane using Escape.
{ Thanks, Sterling. }