It's not a jump back to the 80s exactly. For example, here is an old NT screenshot: http://upload.wikimedia.org/wikipedia/en/c/ca/Windows_NT_3.1...

A lot of it is flat, but they use simple bevels for buttons and other clickable elements, and its reuse on, for example, the buttons in the file manager window, looks tacky because it appears cluttered and overused. Also, excessive lines (e.g. for the window borders), high contrast colors, and small color pallet makes the overall appearance not nice to look at. Also the icon set suuuuucked.