I really, really love the plain panels or mouldings because they are a subtle embellishment, which give depth and interest to the wall while keeping its look light and simple:
They have such remarkable qualities: they are very decorative, they are versatile and can be used in many rooms, they can be made removable (hello to the decorator maniac who wants something new every two months!), they are less expensive than wallpapering a full wall, they are the perfect balance on a bare wall: decorative in a subdued way.
Furthermore, they are ideal to show your personality, both with the paper (or fabric for that matter) and the frame!
I hope you will enjoy these beautiful examples: