Features revealed so far as per the Verge:
Media Enhancements
•Media editing: audio and video; Audio effects, video effects; Slow motion video
•Apps can capture audio/video from themselves
•Media files that are downloaded via IE are automatically added to your phone XBOX Media library. (mp3s are added to XBOX Music, videos to XBOX Video)
•Stereoscopic 3d support
•YouTube Player including HTML5 video support (video playback doesn't have to be full screen) And you get custom UI, so YouTube Closed Captions are in
•Separate volume controls for ringer, media, etc.
•PlayTo support
Networking / Connectivity
•Wi-Fi Direct, a Wi-Fi standard allowing direct peer-to-peer connections over Wi-Fi without an access point
•VPN (Virtual Private Network)
•Bluetooth 4.0 LE (to non-Lumia Windows Phones)
•Feature to mirror Windows Phone screens to TVs and/or monitors
•USB choices
System Enhancements
•Moving closer to universal metro apps (.appx)
•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
•File picker
•Geofencing API for location-based reminders
•‘Battery Power Sense’ info regarding battery usage and percentage remaining. Also lists in order which apps are using the most battery power (nice!)
•Ability to install apps on inserted SD Cards
•Auto-updates for apps (just like Windows 8.1)
•Better backup functionality
•Support for new screen sizes: qHD and QHD (1440 x 2560) and smaller (384 x 640)
•Almost every API in WinRT can run on WP8.1 (WinPRT) these include video and audio transcoding.
•Better Data sense that auto compresses web pages to your specified setting (high, standard, auto, off)
•Ability to add 'trusted' NFC tags/devices so you are longer always prompted to accept content from them
•Screen recorder: Ability to capture screen activity
Apps
•No Facebook integration in People Hub. 'Facebook Blue' app to be installed as default
•Photo hub decoupled e.g Facebook photo thumbnails will no longer directly appear, a hook/link to Facebook photos will be used instead to view the photos within the app.
•Redesigned Xbox Game Hub
•Xbox Music and Xbox Video as standalone apps instead of the Music + Video Hub (allowing more updates to be pushed)
•Updated camera layout, similar to Windows 8.1 including burst mode. You can specify retention period for burst photos to stop them filling your phone.
•OneDrive (previously SkyDrive) with integrated file browser (just like Windows 8.1)
•IE11 improved with WebGL and Normal mapping support. IE Tabs button has been removed and multi task view is now the main way of viewing your tabs.
•IE has a password manager, and most likely syncs with desktop IE
•IE has ability to upload and download files. Downloads are stored in the download folder, files in this can be shared to apps and NFC
•IE has a reading mode and inPrivate browsing mode
•Twitter integrated more deeply into the contacts Hub
•Ability to select your own default messaging app
•Podcast app
•Ability to post to multiple social sites removed. Instead it will work more like the share charm in Windows 8 where you are presented with a list apps you can share the content with, and can only select one.
•Calendar App has a new weekly view
•Bing apps across phone and PC will sync together using MS single sign account
•People hub now has the ability to show contacts who only have phone numbers
•Speed dial
Search and New Services
•Single sign-in for apps (like on Windows 8) with Microsoft Account; sign in will persist across devices + apps with permissions prompt
•Cortana personal assistant that utilises Bing services and FourSquare datasets.
•Bing smart search (similar search system as Windows 8.1)
•iCloud support
Compatibility
•Windows Phone 8.1 apps won't run on 8.0 devices until they upgrade
• Documentation confirms ALL current Windows Phone 8 hardware to get 8.1
Navigation and UI
•Slide down to close apps
•Navigation bar color selection
•Back button doesn't terminate the app instead it suspends the app
•More efficient multitasking
•Filter through installed apps by usage/install date
•Quick settings access (Wi-Fi, Bluetooth) by doing a quick swipe from the top of the screen
•Action Center (notifications) accessed by doing a full swipe from up to the bottom
•Ability for apps to issue Silent notifications, update/delete notifications
• RUMOUR: new ‘large’ tile size
• RUMOUR: mark a Live Tile as read by swiping from the left of said tile
• RUMOUR: Improved keyboard possibly like android ‘swype’ (unofficial tip off from Tom Warren)
• Ability to choose your own start screen background
•Functionality for On screen windows phone buttons that you can swipe away
•Search button is 'frozen' when using 'full screen' apps to prevent accidental launch of Bing search. When the user is: Taking a photo, Capturing a video, playing games the search button will only work if you double tap it. This is following complaints of users accidentally launching search under the scenarios mentioned above.
•Pivots are using slightly smaller typography (basically title text doesn't use up so much screen space!)
•Tweaked transition animations
•WP uses same progress ring as Windows now.
•Three column tile support (probably only Lumia 1020 and 920)
And some rumoured stuff from WPC:
Windows Phone 8.1 new feature changes (subject to change upon RTM)
•Sharing secured WI-Fi Networks with contacts if they have Wi-Fi Sense. They don’t get to see the shared password though
•Social notifications will now show up in Action Center by swiping from the top of the screen
•Operators can now control Data Sense limits remotely
•Operators can also install certain applications once a SIM card is detected
•Along with normal updates, there are now “important” updates.
•Options to always download emails and pictures within them
•Encrypted and signed e-mail support
•Change default SMS app. You can also change which SMS app will show notifications.
•Enterprise enrolled phones can have their passwords changed and remotely locked
•“Write, when I swipe through letters”. That should be self-explanatory
•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
•Office Lens support. Will scan in documents and screens
•The phone will have to update to the latest version before you can restore it to factory settings
•Option to update store apps automatically
•Option to update store apps when only on Wi-Fi
•App Suggestions by location
•Manual Update checks
•Password protected Office document support
•Trusted app list for NFC that will remove the need for asking whether you want to open a particular app.
•In-call speech commands. You’ll be able to say “Call XXXXXX”.
•Double-Tap to unlock your phone (Feature from Lumia devices on Amber Update and newer)
•Quiet Hours
•Double Tap to Power Off
•Notification if charger isn’t of adequate power
•Screenshot button combination is now Power + Volume Up
•Chkdsk for SD Cards essentially
•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
•Voice Narrator for Accessibility
•The Search button will be the new way to open Speech instead of the Start button (Since the newer phones will have the start button on screen)
•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).
•Change the size and amount of tiles on your start screen.
•Wallet supports tickets and membership cards
•App Syncing
•Backups now include application data (ABOUT TIME)
•Screen Reader functionality (Maybe Text-To-Speech for apps?)
•Screen Recording functionality
•Inner Circle of contacts. Up to 40 contacts can be in the inner circle.
•You can favorite photos by tapping a heart icon
•Virtual SmartCard support