The new Gmail Compose Experience is now becoming THE Gmail compose experience as the opt-out and ability to return to the legacy experience options are being sunset.
I'm a big fan of the new compose window because it makes it much easier to continue to interact with your inbox while still working on a message - it essentially works like a chat window. Google recently rolled out the "Default to full-screen" option under "More options". When they did this there was also a subtle change, with big impact, to the 'pop-out' feature.
I learned a great trick at the Google Apps Kansas City User Group. When you hover over the "pop-out" arrow in the upper right of your compose window you'll see you now have two options. Clicking will take you to the full-screen compose window. If you press shift, you'll see the arrow change - click now and you'll be taken to the familiar pop-out experience. I was really excited to figure this out, It's the little things!