The major change in Windows 10 build 10558 is that the Universal/Metro app "Messaging" is back again. You might recall that Windows 8 had a Messaging app which was discontinued. Also, Skype and Microsoft Edge apps have been updated.
Microsoft is working hard on their browser. In this build, Edge comes with the following changes:
•Tab previews. All mainstream browsers have this feature, now Edge has it too.
•Updated interface for the Download manager.
•Updated interface for Developer Tools, which can be docked now.
Users who already installed the leaked build will notice improved browser performance. However, it might be a placebo effect of a freshly installed build.
The new Messaging app should unify all your conversations from various apps and sources like Skype, SMS and so on in one place:
Context menus got updated. They became a bit smaller and got a more refined look:
The Spotlight feature, which changes the lock screen background image automatically using images downloaded from the Internet, is now available for Windows 10 Enterprise users:
This fancy feature was added to Home and Pro editions in pre-release builds of Windows 10, but in the RTM build 10240, it was removed probably because it was not finished. Microsoft is working on bringing it back with the Threshold 2 Autumn update, which is expected in November 2015.
Another change is that with build 10558, the user can finally install apps to an external drive or an SD Card. This ability was locked in previously released builds:
A new security feature was added to the Settings app. It is called "Find my device" and should help the user find his Windows 10 device using GPS/location info if it is lost.
Besides these changes, Windows 10 build 10558 features a bunch of new icons. We have a separate article for icons.
The leak was from Wzor of Russia. The image is not official.