In Windows Phone 8 platform, apps compatibility is always a problem. Although Microsoft emphasizes that apps of WP7.X can run perfectly in Windows Phone 8, some apps still have problem in WP8. An example, Splinter Cell Conviction, which is one of hottest games, has compatibility problem in WP8. And data shows that more than 20 games can’t run in WP8 because of compatibility. At least, compatibility shouldn’t be a problem in WP9. And I think that is not beyond Microsoft’s ability to handle.
About this, Microsoft should learn from Apple. Until now, iOS 6 can still runs in iPhone 3GS. Windows Phone is still a young mobile OS, andi n the start, it's good for Microsoft to provide latest OS experience to Windows Phone users, and that’s also an effective way to gain more market share. But phone like Lumia 800 can’t be upgraded to Windows Phone 8, which is not a so good idea. And if WP9 can run on all Windows Phones which are capable in hardware, undoubtedly it will give old Windows Phone a second chance to attract users.
We had said before that live tile itself is a notification system. But a more powerful and effective notification center is necessary. Users need a notification center to deal with info from Facebook or Twitter quickly and easily. Windows Phone 8 has disappointed many users in notification center, but in the WP9, it's time for Microsoft to provide users a notification center.
After Siri came out, voice system becomes impressive innovation on mobile platform. Compared with Google Now and Siri, the speech platform in Windows Phone is still functionally simple. And if Microsoft wants to improve user experience, a better speech platform in WP9 is a key point apparently.