Thoughtful affordances are the linchpins of intuitive interface design. Dan Wineman has written a wonderful, succinct post demonstrating what they are, and why they’re essential. This is a core concept in UI work, one designers and developers must be well-versed in if they hope to build great user experiences.
Of course this all tracks back to the hot design topic of the moment: iOS 7. For the record, I think Wineman nails it. The lack of thoughtful affordances is the critical stumbling block with the current design of iOS 7. Beyond nagging issues with color, typography, and iconography, it’s the one I earnestly hope Apple addresses before the OS ships.