While it’s not clear how easier it is to create apps that run across both tablets and phones, the inclusion of this support suggests that’s a big focus for Microsoft with this update. Microsoft is also switching to the .appx packaging for Windows Phone 8.1, the same format that the company uses for Windows 8.
In terms of features, Microsoft appears to be readying changes to allow Windows Phone 8.1 users to switch out their default messaging application. A screenshot shows a setting to "receive texts using" different apps. Microsoft previously made a similar change to allow developers to create replacement Camera apps that could be set as the default. Other changes include a new "Battery Power Sense" feature to monitor and track battery life, VPN support, and the obvious rebranding of SkyDrive to OneDrive throughout the OS. The Reddit poster also reveals that, as previously reported, the Music and Video apps have been separated out, and the combined Music+Video app has been removed. There’s also reportedly a new podcast app, but it’s not clear if this will finally support international markets.
Microsoft also appears to be making some changes to multitasking, with developers reporting that the back button no longer terminates the application. Instead, apps suspend in the background just like apps do on Windows 8. While Microsoft is also building a voice-activated "Cortana" personal assistant into Windows Phone 8.1, it appears that the company is keeping that secret as part of this SDK. Developers report that the search button and any Bing-related functionality provides errors, a clear sign that Microsoft has prevented access to this functionality.
Microsoft is planning to release a full Windows Phone 8.1 preview to developers at Build, where the update will be installable on devices. Today's leaks don't detail all of the new Windows Phone 8.1 features, but we'll update you accordingly if additional changes are discovered.