Gmail's new compose view is a handy way to quickly reply to email messages, but for whatever reason it doesn't automatically open up when you reply to an email. If you'd prefer to use the compose view in replies, the Google Operating System shares a shortcut for doing so.
Instead of just hitting the reply button on emails, Shift+click the reply button and it'll open up the reply window in the new compose view. If you'd prefer a keyboard shortcut, Shift+R does the same thing.
Use Gmail's New Compose Interface for Replies | Google Operating System