Microsoft is adding Windows Ink improvements for PC users in this build, including the addition of Stencils, a Windows Ink protractor tool. With this build, inking in photos is supported.
On the camera front, Build 14951 adds a number of features for both PC and Mobile Camera apps. This build adds higher-contrast capture buttons, the ability to set a photo timer from the camera dashboard; the option of accessing the camera roll with one hand; a new zoom slider and more.
Support for Living Images is enabled on Surface Book, Surface Pro 4, Surface Pro 3 and Surface 3 with this build. (Living Images extends still-captured photos with a snippet of video.) There's also faster shot-to-shot support and improved feedback for saving large videos to SD cards in this Insider build.
As with previous Insider builds, there are a number of fixes in 14951, including a fix for some bugcheck bluescreening experienced by some Surface Book and Surface Pro 4 users in Connected Standby.
In terms of "known issues," there's still no fix for those Mobile users who had issues causing phones to get stuck on the logo screen during the boot cycle. Microsoft's advice is to just hang in there during unexpectedly long boot times. On PCs, those with third-party antivirus products installed may not be able to update to this build, and some may experience issues signing into certain apps (Feedback Hub, Groove, MSN News).