Most underestimated WP8 SDK features.
These features coming with the Windows Phone 8 SDK have the potential to drastically change the Windows Phone Store landscape, due to the new freedoms they enable:
1. Ability to add voice control inside an app.
2. Ability for apps to communicate with other apps (Launch Nokia Drive from within a Yelp listing, for instance).
3. Direct3D support for native game coding. Easier porting of existing Direct3D games to WP8.
4. Unity3D support for cross-platform 3D game development.
5. Ability to easily port over WP8 apps to Windows 8 and viceversa.
6. Tap + Send: Ability to enable NFC sharing with other phones and NFC-enabled devices.
Have any more? Feel free to add your own in the comments.