The first thing to notice, and Microsoft have sort of touched on it, but in a very round-about way, is that this build is a forked rs3_prerelease build. What this means and why it is significant is because this indicates that Microsoft are now in the polishing and fixing stage of the Creators Fall Update cycle - they're not concentrating on major code changes now (as is evidenced by the "forking" of this build), and are instead concentrating on fixing the myriad bugs they've introduced (or not fixed) since the original Creators Update.
On the new features front, Microsoft have added a new "Linked Phone" feature, which allows you to link your Android (iOS coming soon) phone to your PC and then "transfer" tasks from your phone to PC. At the moment all you can do is open a web page from your phone on your PC, but it works quite well and quite nicely (even if it does ignore your browser preferences and dump you into Edge).
There's a bunch of Cortana updates, including getting web results without having to open a web browser, and new commands to turn your PC off, restart it etc. all from the delights of your voice. (rumours that this feature was tested by telling the computer to f*** off are completely made up)
As far as fixes go, the biggest, most important one for anyone using a Surface-type device is that, after 3 builds of flakiness, Windows Hello finally appears to be working again! Surface (and other tablet users) were getting rightly pissed off at being told their expensive devices couldn't support Hello, when they perfectly well could! I've tested this and so far* it is working
Microsoft reckon they've fixed the ability for Windows to auto-log in to finish an update in this build - this "feature" was initially part of the Anniversary Update, but it never worked properly - well now it's back again and apparently sorted. (the jury is out until the next build update to test it though)
Lastly, there's the usual skew of Edge updates; some more work on touch keyboards (useful for mobile and smaller tablets, but an utter waste on anything with a screen 10" or larger), plus a bunch of random bug fixes.
Here's Microsoft's full release notes: https://blogs.windows.com/windowsexperience/2017/07/26/announcing-windows-10-insider-preview-build-16251-pc-build-15235-mobile/#dHpkUVWYxdVt6mvo.97