Specifically on the Mac? Tasteful color UI elements (System 7). Unicode support. Multitouch trackpad gestures for window/app management. Dragging data and file

Specifically on the Mac? Tasteful color UI elements (System 7). Unicode support. Multitouch trackpad gestures for window/app management. Dragging data and files between applications (System 7 Pro, IIRC). The ability to run more than one application at once (MultiFinder, but really only standard in System 7). Not having to manually allocate the amount of RAM an app should use (Mac OS X). The zoom box actually being useful (System 7). Being able to move (some) modal dialogs (System 7). Finder being able to do more than one file operation simultaneously (Mac OS 8). Window minimization (Mac OS 8). Menus that don't require you to keep holding the mouse button down (Mac OS 8). Countless others, really -- System 6 is a very different UX. There's likely some element of modernity you'd actually miss.