Microsoft has released Windows Phone 8.1 to developers and you can get it today, if you follow these detailed instructions from WinSuperSite. You can also wait and all Windows Phone 8 devices should get this update sometime this summer.
The biggest new feature is Cortana. A Siri + Google Now competitor that is capable of accessing all your information. Cortana replaces the default search experiences. Microsoft has paid great attention to privacy here. Like any good personal assistant, Cortana keeps a notebook about you. You can use this notebook to control how much information Cortana can access or modify any false assumptions she may have made about you.
The second biggest feature is the Action Center, which is an Android-like notification pull down. Action Center also allows quick access to some settings of your choosing. So if you were tired of going 4 menus deep to turn on/off WiFi, this is great news.
With 8.1 you can better customize your home screen with any picture you like. Live tiles and the lock screen are more dynamic and offer more information than ever before. Finally, DataSense feature, which slows down your data consumption if you’re running out of your monthly data limit, is being rolled out to all Windows Phones alongside this update.