First of all, it's not as one-sided as people are making it out. I was also annoyed when it first came out, but there are some clear benefits. Being able to browse email in the background while composing email is a huge win for my email workload (CTO for a 30-person company).
There are some important philosophical reasons behind some of these changes that I strongly agree with:
First, the small window by default encourages short emails which is always good. The OA seems to glorify long-winded CEO screeds, but by definition the vast majority of emails are going to be short (and if they're not I don't want to work with you), so it's correct to optimize for the short case. The two levels of pop-out (the second of which the OA appears to be completely ignorant of) scale up and out quite nicely.
Second, the hidden headers and formatting options are encouraging simplicity in email. Most email should not have formatting, and arguably most emails don't have CCs, BCCs, or edited subjects either. Putting them behind one extra click is really not the huge usability nightmare people make out. It takes a little while to get used to and then you just subconsciously do the double click, it's not really any harder than finding them in a sea of icons, it's just that previously you were used to that sea of options. The optimization for the common case is done correctly.
For me the new design (after turning on compact view) is a moderate improvement that did nothing to dilute the core value of Gmail which are the labels, keyboard shortcuts/navigation, conversation view and search/filters. Running it inside Mailplane makes it even better.
There are definitely annoyances, but nothing that's even close to driving me to something like mutt, and forget about GUI clients, all of them feel like molasses compared to Gmail.