Strong rumors stating that Windows Phone 8.1 will be announced and shown off at the BUILD conference on April. It will also be made available later that day or week as a preview to anyone that is part of the Preview Developer Update program which anyone can enroll into for free of cost.

Greater than 70% of the API for Windows Phone 8.1 is same as Windows 8.1 so it's looking like that by 2015, it will be 1 app store to rule Windows 8.1 and Windows Phone 8.1.

Quite a few leaks released in the past 2 days from early SDK release to developers and presentation slides.

Every Windows Phone 8 device made by any OEM will get a free upgrade to Windows Phone 8.1

Credit/Reference for the below leaks including photos goes to WPCentral, Twitter, TheVerge and WMPoweruser

New additions and changes coming to Windows Phone 8.1 as per the leaks.

 

Here's the list of features noted by the developer with Windows Phone 8.1 SDK access:

 

• NEW Audio/video transcoding, Hardware accelerated
• NEW Media editing:  audio and video; Audio effects, video effects; Slow motion video
• NEW Apps can capture audio/video from themselves
• NEW Stereoscopic 3d support
• NEW Wi-Fi Direct, a Wi-Fi standard allowing direct peer-to-peer connections over Wi-Fi without an access point
• NEW Windows Phone 8.1 apps won't run on 8.0 devices until they upgrade
• NEW Documentation confirms ALL current Windows Phone 8 hardware to get 8.1
• NEW Dev tools: SemanticZoom, DatePicker, TimePicker
• NEW PlayTo support
• NEW Single sign-in for apps (like on Windows 8) with Microsoft Account; sign in will persist across devices + apps with permissions prompt
• NEW YouTube Player (see lede image) including HTML5 video support (video playback doesn't have to be full screen)  And you get custom UI, so YouTube Closed Captions are in
• NEW File picker (see above image)
• NEW Geofencing API for location-based reminders
• NEW Background tasks: Bluetooth signal strength, Chat message notification, Device connection change, Device use trigger, Gatt characteristic notification, Location, Push notification, Rfcomm connection, System event, Timer
• NEW Slide down to close apps
• NEW Navigation bar color selection
• NEW USB choices
• NEW Updated camera layout, similar to Windows 8.1 (see photo below)
• Back button doesn't terminate the app
• No more XAP, instead APPX
• VPN (Virtual Private Network)
• OneDrive (previously SkyDrive) with integrated file browser (just like Windows 8.1)
• Filter through installed apps by usage/install date
• Select multiple tiles when customising the Start screen [FALSE]
• More efficient multitasking
• Ability to install apps on inserted SD Cards
• Internet Explorer 11 improved with WebGL and Normal mapping support
• Separate volume controls for ringer, media, etc.
• Bing smart search (similar search system as Windows 8.1)
• Xbox Music and Xbox Video as standalone apps instead of the Music + Video Hub (allowing more updates to be pushed)
• Redesigned Xbox Game Hub
• Twitter integrated more deeply into the contacts Hub 
• Facebook account option REMOVED (updated)
• Quick settings access (Wi-Fi, Bluetooth) by doing a quick swipe from the top of the screen 
• Action Center accessed by doing a full swipe from up to the bottom (separate from the Notification Center)
• Silent notifications which go directly to the Action center
• Possibility to mark a Live Tile as read by swiping from the left of said tile
• Auto-updates for apps (just like Windows 8.1)
• Better backup functionality
• Improved keyboard
• Bluetooth 4.0 LE (to non-Lumia Windows Phones)
• Feature to mirror Windows Phone screens to TVs and/or monitors

Things not included in the SDK:

 

• Cortana
• Action Center + Notification Center
• New gesture-based keyboard (Swype)

http://www.wpcentral.com/developers-leak-new-features-windows-phone-81-sdk