A featureless place or thing has nothing that catches the eye — no shapes, marks, or details that help you notice it, remember it, or tell one part from another. A featureless stretch of desert is sand and more sand; a featureless wall is a blank run of paint. People also use it for faces, rooms, and writing that feel the same all the way through. It is a cool, slightly careful word: you are not saying the thing is ugly or broken, only that it gives you nothing to hold on to. Empty is close, but an empty room can still have interesting corners; a featureless one does not.