The original flat designs, Zune HD and the Zune software, Windows Phone 7, Windows Media Center, was incredibly usable.
All those were produced by small design teams at Microsoft, and for, relative to an entire OS, small projects. (Settings aside Windows Phone 7 for a bit, which IMHO actually had very few distinct UI elements.)
Heck Windows Phone 7, to this day, is unlike anything else on the market, It is still going to be more responsive, and look cleaner, than almost anything else out there.
I am not sure why someone decided "flat" means "no button border", that is where I think it all went wrong.
Oh and also people who think flat means getting rid of text! Windows Phone 7 loved text, text was everywhere!