What's missing on all non-Microsoft platforms, as it turns out, is a formalized way to view at least two mobile apps side-by-side on screen. This is a feature that Microsoft added to Windows 8 and then improved dramatically in Windows 8.1, and while many desktop users scoff at its simplicity, it remains a key differentiator. Windows, as I've noted before, is unparalleled when it comes to productivity, even in the mobile world.
Pretty sure Google will introduce windowing support soon in Android, possibly within the next 18-24 months (Android 5.0, perhaps?). It seems inevitable. iOS, on the other hand - we'll see.