Writing a message in Mail no longer opens a form of labelled boxes. The subject sits where an open message's subject does, the addresses are lines beneath it, and the message fills the rest of the pane, with the paperclip, Discard, and Send at the bottom.
Cc and Bcc stay out of the way until you choose them beside the To line. The From line appears when the mailbox has more than one address. Contact suggestions open as a list under the address you are typing, and you can move through them with the arrow keys.
The draft still saves as you write. Closing the message, or opening another one, saves what you typed first. Discard moves the draft to Trash.