New in Craft CMS 3.6
The complete release notes are linked here. Because of the significant changes in the substructure of Craft, I also recommend reading the upgrade guide.
Responsive live preview
Viewports from smartphones and tablets can now be displayed in the live preview so that you can quickly test the content in the other sizes.
Usability
Newly created entries are automatically saved as unpublished drafts, and simple saves only save a draft until the author consciously clicks "Publish draft" or deletes the draft.
Admins can now imitate/test other users without losing their session. For this purpose, a separate URL can be copied to the clipboard and then called in the incognito mode of the browser.
Plugins
Commercial plugins now automatically have a trial license when you install them locally. If these license keys are moved to the production server, admins get the possibility to buy the plugins there, even if allowAdminChanges is disabled.
PHP 8 and Composer 2
Craft 3.6 and all corresponding dependencies fully support PHP 8. PHP minimum requirements for Craft have been increased to 7.2.5.
Composer 2 is now bundled with Craft and drastically reduces the time needed for installation/uninstallation of plugins and the update process.