Chinese language fonts (especially when mixed with English fonts) and input methods. And I don't just mean the default fonts the OS comes with (those these are

Chinese language fonts (especially when mixed with English fonts) and input methods. And I don't just mean the default fonts the OS comes with (those these are better on OSX by a wide margin), but the way font selection and fallback works for GUI elements that contain a mix of Chinese/English. Also switching the system language on the fly.