Part of the "functionality" was UX. The start menu was designed to be easy to use with a mouse. The layout was compact, it was heirarchical and made heavy use

Part of the "functionality" was UX. The start menu was designed to be easy to use with a mouse. The layout was compact, it was heirarchical and made heavy use of hover as a mechanic, and it tended to weight important things toward the bottom left quadrant of the screen, where the user's mouse already was. Metro is designed touch-first: full-screen, spread out, no hover events, heavy use of drag and swipe. Yes they fill the same basic need with the same fundamental abstraction, but on a traditional desktop with a mouse as the main pointing device using Metro is a much worse experience.