A healthy tension between Design and Engineering seems to be the best - sometimes the best UX isn't the most efficient and engineers need to get pushback on that.
Similarly, sometimes the "coolest" UX isn't technically reasonable, and the Designer needs to come back with something else.
The buttons, for example, seem completely arbitrarily placed and inconsistent. Check out the difference between compose or replying to an email. Or creating a calendar entry or the full page editing calendar screen (and the two are the 'opposite' way round, it's totally fubar). It seems to be according to some sort of an attempt at 'page balance' that got abandoned in more complex elements to my admittedly untrained eye.
A red button seems to be intended to be the primary action on a page but again each part of the system is totally erratic on whether this convention is followed or not. Add a calendar event? No. Edit a calendar event? Yes. Reply to an email? No. Compose a new email? Yes.
Why do three little dots appear next to the check boxes when you hover over an email? How does that help at all? It just seems to distract my eye and doesn't actually help me distinguish whether I'm about to check the right email at all, the usual reason for hover styles. It's just pointlessly pretty that's turned into distracting.
The more I use it, the more confused I am exactly how the hell it got signed off. Bizarre.
The change between the 2 concepts should surely have told someone that the icons simply didn't work on their own - nobody knows what the "spam" icon means without a text popup when you hover over it.
That said, I personally like most of the new Gmail quite a bit more. It's probably because I get everything done with keyboard shortcuts anyhow and so care more about how it looks that how the buttons actually behave :P.
I have no insight into Google's process, but I'm curious if others run into cases where designers have limited sensitivity to UX (and ignore feedback about it)?
Maybe DRY should also stand for "Don't Reduce Yourself." (As in avoid excess reductionism.) Not only should every significant idea only be expressed no more than once, every significant idea also needs to be clearly expressed at least once -- and not expressed as an oversimplified caricature.