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).
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.
I’ve still stuck to using Sparrow and Mailplane for my email, but when I have to move back, this is going to be immensely useful. Seems to be working only on non-Google Apps accounts though.