The WordPress 4.4 field guide is available and covers all of the major features in WordPress 4.4. The guide explains what the features are and more specifically, links to posts that explain how they work.
While it doesn’t cover every single change, it gives developers and site maintainers an opportunity to learn and understand the major features before WordPress 4.4’s release.
If you haven’t tested your plugins and themes with WordPress 4.4, now is a great time to do so. In testing WordPress 4.4 betas on WP Tavern, I discovered two broken plugins. I notified the developers and they quickly released an update addressing the issues.
WordPress 4.4 is scheduled for release in December.