It is quite possible for wall art to look cluttered and chaotic if too many pieces on the wall are not similar. This is why as a rule of thumb, you need pieces with some type of similarity grouped together. It could be an era or color link, or subjects such as oil-painted ocean scenes, or watercolor flowers, etc.
Like-colored frames are another way to make all of the pieces come together, whether the frames are in gold, black, or white. Cheap oversized wall art should also be hung at eye level and work outward to create a gallery effect.
Art ultimately is personal; however, the way you hang it does involve a few concrete rules. As long as you keep the theme and color of each piece of wall art consistent, you'll create an effect that is pleasing to the eye.