Gmail Labs graduated three features, Superstars, Nested Labels and Advanced IMAP controls while retiring the Google Search box. Superstars is a feature where Gmail offers more stars than just the regular. These stars can mean different things, one star can be used for special conversations, another star can be used as a visual reminder to follow up on certain emails. You can choose which stars you want from the settings and they rotate as you click the star icon in your inbox.
Superstars (above) and more IMAP controls (below)
Nested Labels is a feature where Google allows you to organize your labels hierarchically. Nested Labels are available for everybody along with a few features like a sticky collapse/expand state. To start using them, you can either create a new sub-label from the dropdown menu on the left hand side or move an existing label under another one using the edit option. Advanced IMAP controls are for people who use Gmail on mail clients like Outlook, Thunderbird and Apple Mail. You can now sync selected labels, and limit folder sizes.
The retired feature was the Google Search box lab that was redundant with the “search the web” button that's already in Gmail.