A User Interface is considered engaging when the experience of using it is enjoyable and easy. Its visual design, graphical consistency, color scheme and interactive elements all play vital roles in a user’s immediate perception. Less obvious aspects of the interface such as the layout and readability of the text also contribute to an engaging experience.
These UI characteristics must be directly tied to the UI system in place. What is enjoyable and engaging for repetitive work done for hours on end is different than what would be satisfying for a product that gets used in smaller intervals. What is important is that the design meet the divergent expectations and goals of the end users.