Microsoft has posted the official Windows 8.1 Update 1 changelog.
The list of improvements include:
OS version: 8.10.14141.167 or 8.10.14147.180
Folders. Folders let you organize your Start screen the way you want. Just push one Tile on top of another one to create a folder and get going.
Messaging. Combine multiple text messages into one, and then forward them to someone else.
Apps Corner. Apps Corner lets you specify apps other people can use on a Windows Phone. Great for individuals and any-sized organizations who want to provide only the essential apps for others to use for work, school, or any other reason.
Alarms. Now you can customize the snooze time for an alarm, and then enjoy a little more shuteye for the time you want.
Accessory apps. Use accessory apps to get notifications from your phone on your smart watch, active phone cover, fitness tracker, or other kinds of accessories.
Internet sharing. Now you can share your cellular data connection over Bluetooth, so you can get an Internet connection on more kinds of devices.
VPN. VPN now supports L2TP, which lets you connect to more VPN services. Whether you connect to a VPN for work or personal use, you can do it on your Windows Phone.
Narrator. Narrator now has touch typing and a way for you to turn off hints for controls and buttons if you don’t want them read aloud. Some other improvements to Narrator help you unlock your phone a little more quickly, find and use the Back, Start, and Search buttons more easily, and tell you when your phone screen is on or off.
The list is of course not comprehensive, and we know some features such as improvements in IE11 and OEM specific changes are not included, but the update is certainly not insignificant, as we have heard some bloggers calling it.