There's not so much that's new in this release, but there are a *hell* of a lot of fixes for stuff that broke in the previous, 17035 release. One new feature of note though is the SDR brightness on HDR hardware control. As Microsoft put it:
Adjust the brightness of SDR content on your HDR display: Starting with Build 17040, Windows now lets you adjust how bright SDR content appears when running in HDR mode on the desktop. On an HDR capable system, you will see a slider in the “HDR and advanced color settings” page under Settings > System > Display. This is one of the improvements to HDR image quality that we have planned based on feedback from Insiders.
In addition to this, there's a few more input and handwriting tweaks, plus those aforementioned bug fixes. Here's the complete listing: Windows 10 17040 release notes