I think what you fail to understand is that Google are building a tool for the masses and I would bet that they have analysed what features people actually use and most of the time the majority of people don't change the subject or modify the recipients or format the email.
In short - they're catering for the typical user, and the typical user doesn't use all the features that the OP thinks they need.
Ever wonder how bland, feature barren, largely useless lowest common denominator shit that nobody really seems to like, or identify with can come to dominate design of software products?
My theory is that the assumption there is some kind of meaningful "Average user" which a product is then built for ultimately destroys utility in software.
If you have 50 features that on average 1% of people use, you can easily reach the false conclusion nobody cares about these features, when on aggregate 90% might use at least one of those features. Thus in aiming for the average, you haven't designed for anyone at all.
Yes, I think you're right. When they brought in priority inbox, then the split tabs for promotions, etc, I couldn't switch them off quickly enough. I aim to get my inbox to zero, so these just got in the way. I imagine that the vast majority of users let their inbox pile up into the thousands, so these features would be a huge help.
Thankfully these examples are optional, unlike the author's qualms.
I'm like you. My work Gmail account currently has about 30 messages in the inbox but my personal Gmail has 2573 (in the priority inbox alone). The use cases for work vs personal are so different it's hard to fathom any single app that would handle both equally well. That said, I get enough vendor spam at work that I'd really welcome a "Promotions" inbox that force-separated the chaff. And, since we use G+ at work, too, I'd be cool with a Social inbox, also. I thought I'd hate that part of the redesign but it's turned out to work pretty well. The lack of formatting buttons in the compose box, on the other hand....
Do you have information we don't? I hardly think any user testing at all.
I hate I when organisations like Google, who have no real personal contact with end users, just do a redesign without consulting the people who actually us their products. It's the Alan Day school of design, and it is totally unnecessary.
Are big editing area or the option of editing subject really such esoteric, geeky features beyond the concern of an average user? What has become of us if so?
All the sub average users I've talked to about the Gmail changes dislike what they've done. I don't believe for a second that the changes they've made are positive for the so called masses. Just my opinion.
In short - they're catering for the typical user, and the typical user doesn't use all the features that the OP thinks they need.