Microsoft today has published a list of the changes and additions in Update 1 for Windows Phone 8.1. It's always nice to see things spelled out this clearly, but the Microsoft list is unfortunately quite incomplete as well.
You can see Microsoft's list of changes on the Windows Phone 8 update history web site. Microsoft notes that Update 1 will increment the Windows Phone build number to 8.10.14141.167 or 8.10.14147.180, depending on your device model, and will add the following changes and new features:
Folders. Routinely called "Live Folders," this feature "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." I just wrote about Live Folders—sorry, "folders"—in Windows Phone 8.1 Update 1: Live Folders.
Messaging. Now you can combine multiple text messages into one, and then forward them to someone else.
Selection. With improved selection, you can now select and then delete multiple calls (in the Phone app), messages (in Messaging), or contacts (in the People app).
Apps Corner. This new app 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 now do it on your Windows Phone. (Note that this description is a lot more restrictive than I was originally told. I'm looking into how one might use this functionality as a consumer.)
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.
So, that's all very nice. But as noted, it's also incomplete.
For starters, Internet Explorer 11 got a massive update in this release, and I wrote about that recently in Windows Phone 8.1 Update 1: Internet Explorer. There's no mention of the many Cortana improvements either, or the smart cover support, the new supported resolutions and screen sizes, dual SIM support and much more. Check out my Windows Phone 8.1 Update 1 Preview for a more complete list.
Long story short, this Microsoft list isn't a "changelog," it's just a partial list of new features and some of the changes to existing features. But many of the biggest changes aren't even documented, which is kind of crazy.