- buttons are labeled and placed correctly, and respond to expected input (including secondary focus and secondary action on MacOS, for example)
- dropdowns/selects behave correctly, and respond to expected input (for example, you can type to select in MacOS dropdowns).
- windows have OS-defined areas for drag/resize, for titles, for toolbars etc. They also appear correctly in the OS's window management tools (app switchers etc.)
- text inputs use the OS-default shortcuts and have access to OS-provided tools (e.g. spell checker)
- controls and windows respond correctly to keyboard and mouse. E.g. for a while Apple's own Catalyst apps didn't have the standard Cmd+Q to close the app. Many custom modals do not dismiss the modal on Escape
- context menus are standard in the places where you expect standard context menus. Well, app menus, too.
And the list just goes on and on.