Here are just 4 things that it breaks for me (note: copied from elsewhere):
1) When you hit "forward", it hides the subject box, even though the majority of the time when I hit forward I want to delete the "Fwd:" from the subject line. Especially when sending a form email. This resulted in a couple of embarrassing emails before I changed back to the old compose.
2) I often want to create a big list of emails that I want to review before hitting send. The new compose replaces each email address I type with the persons name! For a lot of people I do business with, I have both their business and personal email address. But now I can't tell which one I'm about to email. What the hell Google?
3) Attachments are now an object within the email. But often I want to forward something with an attachment, but clear out the text in the email. Well now when I do select-all, it deletes the attachment also.
4) The compose email doesn't cover my screen, even when I pop it out as its own window. When I'm drafting an email, I don't want to be able to see distractions. If other emails pop up while I'm editing it, that can make me lose my train of thought. Worse, there are no options that I can set to change this.
Additionally, with this interface, it could be more integrated with hangouts and cross google's services (maybe easier send email pop up on andriod tablets?).
The "pop out" choice is a poor one since it turns Compose into 3 operations: Press compose, press the pop-out button, press maximize on the new window.
(as for the chat box, I use Minimalist, and my Gmail UI is trimmed of anything extraneous).
the problem with google launch for that feature is that they DID NOT give the choice.
So, people that is not slow and know how to use tabs effectively, got their writing space crippled. and it became a pain to actually see the email i wanted under the in-page-popup composer. because you know, some people have 10inch screens.
at least now they made it available to go back to the old format. ...that will give me a few more months to look to a gmail alternative.
The interesting truth is that Zenter was the first. I know because they demoed it to me in 2007 shortly before Google acquired them.