Key features of a responsive mobile app include adaptability to different screen sizes, user-friendly navigation, and fast loading times. A responsive design ensures that the app displays optimally on various devices, enhancing the user experience. Additionally, incorporating touch-friendly elements and maintaining clarity of content is crucial for usability. Smooth interactions and quick responsiveness not only increase user engagement but also contribute to positive app store ratings. As mobile internet usage continues to rise, responsive design is vital for retaining users and achieving success.