XP and Vista look completely different. Not only was Aero introduced in Vista, the taskbar and Start menu were redesigned.

Vista and 7 look very different. The UI was cleaned up a lot, the taskbar was redesigned again, Live Thumbnails and multitouch support were added.

Let's compare that to OS X: most people won't be able to tell Mountain Lion (released 8 months a go) from Leopard (released in 2007), just by looking at the desktop.

Here are two screenshots. One is Mountain Lion, the other is Leopard. See if you can tell which one is which. http://i.imgur.com/VDVPvV2.jpg http://i.imgur.com/0fG7xNu.jpg