Yesterday, Apple released iOS 12, watchOS 5, and tvOS 12 to the world, supporting every device that received last year's updates except for the first-generation Apple Watch. As usual, the first minor update to follow is now available in beta form.
Unfortunately, while the first developer betas are out, Apple hasn't published any release notes. The updates themselves contain bug fixes, and all of the release notes pages say "The page you're looking for can't be found."
We can take an educated guess as to what's included, as Apple does this every year. Basically, it's usually the features that didn't make it into the first update. That means that you'll probably see a return of group FaceTime calling in the beta, even if it still doesn't make it into the final iOS 12.1 release.
You can also expect to see support for the new emojis that Apple announced earlier this year, and that are part of Unicode 11.0. Those usually show up in the "point one" update.
Finally, watchOS 5.1 likely has the new ECG features that are in the Apple Watch Series 4, as they won't ship with the device at launch.