Coding enhancements, Customizer enhancements, more widget updates, and site building improvements.
Things to note
There are a lot of great improvements packed in WordPress 4.9, including one of my favorites – error checking when working directly with files through the WP Admin. One of the scariest things in WordPress is editing plugins/themes directly through the admin interface the next scariest thing is making a change, getting the white screen (an error) and then not being able to access the site. Fortunately for the people that make these changes via the editor – there is now error checking in place so that any errors are caught/identified before the save – saving all of us white screens, and stress.
There are also a number of enhancements to the Customizer so making your site exactly how you want it is easier than ever. You can even save “drafts” of design changes, preview them, share the preview links, and collaborate before you release changes and go live. Speaking of going live – as of WordPress 4.9 you can schedule your new design to go live at a specific time/date if you choose.
All in all – there are a lot of great changes in WordPress 4.9. You can read more by looking at the release notes: https://codex.wordpress.org/Version_4.9
If you have not done so already – download the update or simply use your existing installation of WordPress to update your site. Either way – this is an excellent time to make a backup copy of your site(s) before upgrading.