Hierarchical labels and the Message Preview are the two powerful and useful experimental features of the Gmail Labs. If you’ve used the Greasemonkey, Folders4Gmail and Mihai Parparita’s Conversation Preview, consider that these two are the similar functionality to the new Gmail Labs experiments.
In such a way, Nested Labels is also a cosmetic change that lets you create the Labels, which are displayed hierarchically. You can view the Linux as a subfolder of Mailing Lists when you enable this experiment and create a label like Mailing-lists/Linux. At the same time, all the other places that let you interact with the labels show the label as Mailing-Lists/Linux.
Google says, “You can create Complex hierarchies of labels if that’s the way you like to organize the mail and you can expand/collapse labels to save the 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.”
Whereas, Message Sneak Peek is able to display a small preview of a conversation to read the first sentence with opening the conversation quickly. Just, right click on the conversation from the Inbox or from another view. If you’ve finished viewing all the message, Gmail will show the last message.
Moreover, there is a Keyboard Shortcut is available for displaying the preview pane for the currently selected conversation (You have to press the “h”). Therefore, you can navigate to the previous or next conversation using the K and J. If you want to dismiss the pane, you can do it using the Escape.