4.5 includes several new features that need to be tested before the software ships, including a responsive preview of your site in the Customizer, inline link editing, improvements to comment moderation, theme logo support, and more.
Several under-the-hood changes for developers also need to be tested:
- selective refresh
- backbone and underscore updated to latest versions
- embed templates
- new WP_Site class
- script loader
You can view the full list of changes here.
To help, download the beta and play around with the new software. You can report any bugs in the Alpha/Beta area in the support forums.