Microsoft send private invitation to selected developers on last February for Windows Phone 8.1 beta SDK release. As it is a developer SDK, and we know the SDK never revel all consumer end features, but something learned and got from that release. Thanks for the anonymous developer share the screenshots and details about the same on next day itself of the private beta release.

Windows Phone 8.1 Code Named as “BLUE

Note: The features listed below is as per the beta release of Developer SDK of WP 8.1, I can't make sure all the features listed here will be available on the consumer release.
  1. Developer
    • XAP replaced by APPX
    • ​​Audio/video transcoding that’s hardware accelerated
    • New dev tools including: SemanticZoom, DatePicker, TimePicker
    • Actionable notifications for apps
    • Silent/ghost notifications for apps
    • Background tasks added like Bluetooth signal strength, Chat message notification, Device connection change, Device use trigger, Gatt characteristic notification, Location, Push notification, Rfcomm connection, System event, Timer.
  2. User Experience
    • File picker (May be a lot useful)
    • Kid’s Corner – Password won’t be displayed (bug fix)
    • Live Tiles – mark as red by swiping from the left over the Tile
    • Live Tiles – Three column support (maybe)
    • Navigation bar for phones without physical keys on the front
    • Notification if charger isn’t of adequate power
    • Quiet Hours – Disable notifications during pre-selected timeframe
    • Screenshot button combination is now Power + Volume Up
    • Search button will be the new way to open Speech (aka ‘Cortana’)
    • Separate volume controls for ringtones/notifications and music/media
    • Set default Voice Navigation app for the OS
    • Swipe down to close apps in multitasking view
    • Voice Narrator for Accessibility
    • Start screen backgrounds with parallax motion
    • Back button no longer closes apps, instead it suspends them
    • Double-Tap to unlock your phone (Feature from Lumia devices on Amber Update and newer)
    • Double Tap to Power Off
    • Favorite photos by tapping a heart icon
  3. System
    • The phone will have to update to the latest version before you can restore it to factory settings
    • Trusted app list for NFC that will remove the need for asking whether you want to open a particular app
    • Virtual SmartCard support
    • Wallet supports tickets and membership cards
    • Windows Phone 8.1 apps won’t run on 8.0 devices until they upgrade
    • Bluetooth 4.0 LE
    • Chkdsk for SD cards to ensure card health
    • Geofence monitoring support. Geofence is a boundary around a place of interest to you. Location services can monitor geofences to tell your apps when you’re entering and leaving them. This would be really good for IFTTT
    • Miracast, PlayTo, mirror and project via wireless display or USB-out
    • Mouse and keyboard support (HID)
    • Stereoscopic 3D support
  4. Action Center
    • Notification center – Alerts, non-Toast notifications, update history, more
    • Quick-access to customized actions, including Wi-Fi, Bluetooth, Plane mode, etc.
  5. Camera
    • Burst camera mode, including auto-deletion of bursts
    • New layout
  6. Calendar
    • Google calendar support, with multiple calendars
    • Weather in calendar
    • Week view in calendar
  7. Apps
    • New Store design and features
    • Option to update store apps automatically
    • Option to update store apps when only on Wi-Fi
    • Single sign-in for apps (like on Windows 8) with Microsoft Account; sign in will persist across devices + apps with permissions prompt
    • App Suggestions by location
    • Backup of App + Game data to OneDrive (if developer enabled)
    • Facebook app comes with OS, updates independently, hooks into OS
    • Filter through installed apps by usage/install date
    • Manual Update checks
    • Apps can capture audio/video from themselves; be limited for copyright concerns
    • App and Game data stored on SD cards
    • Apps on SD Cards are only allowed from ONE originating phone. If it detects that apps from another phone are installed on the SD Card, then it will ask to delete those first
  8. Browser
    • Internet Explorer 11
    • IE 11 to save and remember your password for websites
    • JavaScript and HTML for app development
    • Upload files through Internet Explorer 11
    • Tabs now show up individually in multi-task window
    • WebGL and Normal mapping support
    • YouTube Player including HTML5 video support
  9. Email
    • Encrypted and signed e-mail support
    • New Email sync options with accordance to usage patterns (i.e. If you get a lot of mail on particular days and if you don’t check mail every day).
    • Option to always download emails and pictures within them
  10. Office
    • Rebranding of SkyDrive to OneDrive
    • Certificate management to enroll, update and revoke certificates for user authentication
    • Enhanced MDM policies to lock down functionality on the phone for more enterprise control, in addition to richer application management such as allowing or denying the installing of certain apps.
    • Enterprise Wi-Fi support with EAP-TLS
    • Enterprise enrolled phones can have their passwords changed and remotely locked
    • Password protected Office document support
    • Office Lens support for scanning documents with camera, import into Office with OCR
    • VPN support built in
    • S/MIME to sign and encrypt email
    • Access to corporate resources behind the firewall with app aware
    • Auto-triggered VPN
  11. Media
    • Audio/video transcoding, Hardware accelerated
    • Media editing:  audio and video; Audio effects, video effects; Slow motion video
    • Xbox Music and Xbox Video as standalone apps instead of the Music + Video Hub (allowing more updates to be pushed)
  12. Messaging
    • Change default SMS app. You can also change which SMS app will show notifications.
    • Mute Thread – Optionally silence text threads, no longer receive notifications or Live Tile updates
  13. Keyboard
    • General keyboard enhancements, show emoji when typing
    • Swipe keyboard option
  14. Contacts
    • Contact list sorting of those with phone numbers
    • Call duration (press and hold reveals a “details” menu in call history)
    • In-call speech commands. Say “Call” then ‘number’
    • Inner Circle of contacts. Up to 40 contacts can be in the inner circle.
    • Operators can also install certain applications once a SIM card is detected

Well, let us wait for the final release.