I don't think anything in the software world has ever pissed me off as much as the fucking ribbon. "We've run out of ideas as far as the UI is concerned - just throw everything up there somewhere, menus, toolbars, whatever". A close second was their genius idea of hiding unused menu items so the locations of the items you do use are constantly changing.

I don't use MS products any more but my 90 yo parents do and it's a fucking nightmare trying to help them with stuff. MS Office is certifiable elder abuse.