Microsoft's Windows 10 Build 10052 solves phone-bricking problems
The major problem that the new build solves is an issue where some users--including owners of the most popular Windows phone, the Lumia 520--suffered catastrophic failures that "bricked" the phones when they tried to roll them back to Windows 8.1. Microsoft said it has solved the bug on Lumia 520 phones, with an update to the Windows Phone Reset Tool.
The new build doesn't contain any new features, just bug fixes. According to Microsoft, these are the things the new build addresses:
Microsoft didn't release any additional information about future builds or any features they might include.
Why this matters: We've said before that Windows 10 for phones should be left to those who have some spare hardware lying around, not your primary phone. The safety net that Microsoft put in place, the Windows Phone Reset Tool, apparently can be subject to the same bugs. Fortunately, Microsoft has patched that too. Still, Microsoft users will be less willing to risk a phone to test out Microsoft's beta OS after this.