It will come as no surprise that pre-release software isn’t complete, reliable or even usable. That said, most of the Windows 10 Technical Preview builds have been pretty solid so far (though I probably still need therapy for that November build). Here’s a list of what Microsoft says you should look out for in the new build.
Boot selection menu. Microsoft says that you may see a boot selection menu each time you reboot your PC. But this behavior is only temporary.
Xbox Live enabled games that require sign-in will not launch correctly. A fix for this will be distributed via WU shortly after the release of Build 9926.
Battery icon shows on lock screen of PCs without batteries. This was in previous builds as well, actually.
Tiles on the Start menu show truncated app names (such as the Windows Feedback app).
Remote Desktop has some painting issues that result in tiled pixelation.
Connected Standby enabled devices like Surface Pro 3 may experience shorter than expected battery life. LOL. Well. There you go.
Cortana reminders can’t be edited and more complex reminders might not get created. The first reminder you create in Cortana in this build might not pop up, but subsequent ones will. Completed reminders also don’t move to the History page in Cortana’s Notebook.
Music app disappears. The Music app will disappear if minimized within 16 seconds of launch. Just leave it in the foreground for 20 seconds or so and it should work just fine, Microsoft says.
Start menu fails. Occasionally, the Start Menu is improperly registered and will fail to launch.
Microsoft notes, too, that it may release updates to fix some of the most prevalent issues, while other issues may be fixed in later builds.