Plain walls can look awfully boring in living rooms, dining rooms and bedrooms, and there are so many different options out there today, so there really are no excuses. Making good use of empty space is part and parcel of interior design, and an empty gap on a kitchen wall may only need a clock or a painting to look 10 times better.
You can buy cheap canvas style prints to cover large areas, while framed photos or paintings are always a good option. Office areas or bedrooms can look nice when there are inspirational posters on the walls, while funny signs or vintage advertisements can be nice touches.
Make sure you liven up your walls with different wall hung items, or opt for shelving if you need more storage space. A plain wall is sometimes nice, if the wall is a feature of the property, like an exposed brick wall, but otherwise it can be wise to liven it up ever so slightly, and there are many options out there.