A well-designed interface often feels obvious because it lets people complete a task without thinking about the interface.
When an interface works well, I tend to notice what I came to do rather than how the product arranged it. The right action is easy to find, the labels make sense, and the next step does not need much explanation.
That can make good design seem effortless, though the effort has not disappeared. Someone had to understand where people hesitate and remove the unnecessary choices or confusion. The result may look plain because it does not call attention to itself. In software, that is often a better outcome than making every interaction feel clever.