You might notice that the 'Compose' screen on Gmail looks somewhat different today.
According to an article published on Google's official Gmail blog, the newly introduced 'Compose' screen was designed to be faster and easier to use compared to the old interface which 'covers' the entire inbox. In a nutshell, hitting the 'Compose' button would fire up a pop-up window at the bottom right corner of your screen, much like chat screens on Google Talk, albeit in a larger window.
This makes sense, especially if you need to refer to another email in the midst of composing a new one. Considering that many of us have to save the draft message, open up the old email, and then reopen the draft message in order to complete the email, the new 'Compose' layout does help speed up the process in this aspect.
Besides facilitating easy references, the new 'Compose' also makes it easy for you to keep an eye on incoming emails. Again, much like chat screens embedded in Gmail, users can minimize the 'Compose' window (and even multiple windows) to work on them later. According to Google's blog post, it's also easier to insert inline images with the reworked 'Compose' window.
Furthermore, you'll now see profile pictures of your contacts as you're adding recipients to an email message. They'll appear in the suggested lists of recipients as part of Google's autocomplete feature. In addition, users can drag-and-drop recipient chips between the "To", "Cc", and "Bcc" address fields, instead of having to copy and paste them.
Inline replies will also feature a similar layout as the new 'Compose' screen. According to Google,"replies take up much less vertical height, intelligently expand to fit your content, and always keep the recipients and other controls in view no matter how long your message gets".
Source: Gmail Blog