Variety creates interest it can be used to move the eye through a painting and its complexity is endless. To see variety you must make yourself aware of the differences between pictorial elements that share similar traits but have a unique quality that creates their individuality. A successful artist can reveal subtle differences that most people over look.
Variety can be created in a number of ways within your image, you can vary the size (contour, height and width) of elements in your scene you can vary their coloring or their angle or spacing to each other. All of these changes add interest and they can be designed into the painting once an artist learns to see them.
Variety can also be added through brushwork and surface quality. Some passages can be painted thinly some with thicker paint. The calligraphy of the brushstrokes can be varied so no two are the same in direction or size.
Edges are another area that gives an artist many opportunities for variety. Edges can be soft or hard optically or they can be physically blended or left sharp and unblended.
Adding this kind of interest insures the painting has lasting appeal beyond the attention of the image itself. Although the subject matter in the image is what draws many people to a particular painting eventually it is the structure and handling of the painting that gives a work power and separates it from current fads or cliché. It is the strength of the design that allows it to stand the test of time.