Very interesting that you should bring flat UIs up. I monitor my father's inbox[1]. I noticed earlier today he got an email to confirm the creation of a Microsoft account he needed. Instead of clicking on the flat blue "Verify xxxx@gmail.com" button to confirm his email address, he replied to the email saying "verification". It then bounced, predictably, and confused him even more.

I haven't spoke to him yet, but I'm guessing that the flat blue button with some text, in the email didn't seem "clickable" to him.

Flat UIs do indeed seem like a serious step backwards in usability.

[1]Phishing protection