if it is a good design, this is a positive feature.. constantly breaking a good UI just to be hip and cool is not.
I'm not against new designs if they bring improvements to the table, but mindless bandwagon jumping is a waste of time.
Geary reminds me of the reddit redesign or some other UI that Apple or Google would create. Information density and convenience where obviously sacrificed for someones idea of aesthetic.
They removed button labels for no apparent reason other than to confuse you with duplicate icons as far as I can see in the screenshot. Looks like it takes at least 2 clicks to sort emails. I'd bet that probably the most annoying thing though is the lack of features and options.
It looks really nice though and if it fits your needs, I'm sure it's great. But those of us who like desktop software and not things like Gmail probably won't.
Everyone has their own preference. I just kinda like the idea of more lightweight software. Thunderbird is based on a lot of old Mozilla code, if it were redone solely off of Servo that would be something, servo for message rendering but the rest of the UI can be native or just similar and not too JS heavy if the UI is some Servo HTML UI.
Thunderbird, Evolution and another clients UI looks like designed 10-15 years ago.