Google announced today in a brief Google+ post that Gmail users can now add labels and stars to messages from the new compose box.
Add labels and stars in the new compose
You can now add labels and stars to messages in the new compose by clicking on the More options menu in the bottom right and hovering over Label.
553 
99 Powered by socialditto
Google revealed the new compose feature back in October in preview. You should see it as an option from your inbox if you haven’t already switched over.
“How many times have you been writing an email and had to reference something in another message? Saving a draft, opening the old email, and then reopening your draft wastes valuable minutes. The new compose pops up in a window, just like chats (only larger),” said product manager Phil Sharp. “This makes it easy to reference any other emails without ever having to close your draft. You can even do a search or keep an eye on new mail as it comes in. And because the compose window works the same way as chats, you can write multiple messages at once and minimize a message to finish it later.”
“The new compose is designed to let you focus on what’s important: your message,” he added. “The controls are still there when you need them but get out of the way when you don’t. We’ve even added some new features like the ability to easily insert inline images and have more to come.”
Google said when it launched the new Compose, that it would be adding some finishing touches over the coming months. I guess today’s announcement falls under that umbrella. Perhaps we’ll see the changes widely implemented in the near future.