EIS (Electronic Image Stabilization) requires a lot of computing power, which is why many phones put an upper limit of 1080p/30fps. The Snapdragon 835 has enough grunt to pull it off, though.
The rest of the changelog includes Wi-Fi connectivity improvements (again) and standby battery enhancements (it doesn't say if Energy Aware Scheduling is enabled). There's also a new font (OnePlus Slate), Android Security Patch July 1st 2017 and a newer Google Mobile Services package.
There are bugfixes as well:
Note that this update is being distributed incrementally over the air. Today, only a lucky few will get it, but the rollout will pick up speed in the next few days.