Backward compatibility to 7
To gain any traction, they urgently need to backport the Windows Runtime and App Store to run on Windows 7. If I'm developing software for companies, I can't possibly tell them "Here's your new app, by the way you need to upgrade your entire estate to Windows 8." Most of my clients have only just migrated from XP to 7; they're in no hurry to go through the same hell again.