Another complaint about Windows 8: The Metro environment was divorced from the traditional Desktop, so you had to switch back and forth depending on the apps you were running. Windows 8.1 added the ability to put Metro apps side by side, but didn't address the underlying issue.
Windows 10 will let Metro apps run in Desktop windows like any other app, getting rid of the duality of Windows 8. That leaves the question: How's a Metro app different from a Desktop app beyond the look and feel?