Tom Warren has posted some more details on Windows Phone 8.1. Sources who are in the know say that 8.1 will have a notification center that users will access by swiping down from the top of the screen just like in iOS and Android. Similarly a shorter swipe from the top will give you quick settings so users can turn things on and off like WiFi etc.
Cortana will be in as well, a mix of Google Now and Sir. Multiple volume controls are also in 8.1 finally so you can set ringer and media to different levels. One big change is that they're going to be splitting out music+videos from the OS and making them separate Xbox music and Xbox video apps so they can be upgraded more often. The team is also looking at doing the same with more apps while also allowing third parties like Facebook and twitter to modify the people hub more, probably other social networks as well.
They're expecting MS to reveal 8.1 at BUILD in April.