Real download manager in Safari, much improved and rethought automation, very carefully designed Dark Mode (with usual Apple details you may have missed - read Federico Viticci’s iOS/iPadOS review if interested), 3D/haptic touch improvements, noticeable performance improvements, external drive support (on iPhones, too), DualShock 4 and Xbox controller support alongside Apple Arcade, more natural Siri... and that’s not counting numerous improvements in maps, keyboard, ARKit, HomeKit, visual and functional tweaks, Apple Sign-in, etc.
Just for iPhones alone (not even mentioning the insane iPad features) this is one of the most feature-rich, packed releases ever. Literally every aspect of the OS has been improved and there are a bunch of new features.