Characteristics of effective user interfaces include clarity, consistency, responsiveness, and user-friendliness. A well-designed UI enables users to navigate easily without confusion, ensuring that essential features are readily accessible. Consistency across different sections of an application helps users build familiarity, while responsive design adjusts to various screen sizes for optimal usability. Aesthetic appeal combined with intuitive elements promotes a positive user experience. This encourages user engagement and satisfaction, ultimately driving the success of the application or website.