One of the most notable things about this isn't even mentioned - and that is that, perhaps, this is the start of Google rolling out their Material Design princi

One of the most notable things about this isn't even mentioned - and that is that, perhaps, this is the start of Google rolling out their Material Design principles to their existing properties. Inbox uses MD heavily, but it was brand new and built from the ground up using it. As far as I know, Contacts is the first piece of the website puzzle to implement it after the fact. EDIT: I completely forgot about Google Docs, Sheets, Drive, Account Settings, Alerts and probably others all going Material too.