There are many things that constitute a piece of art. I've seen a lot of art and no matter who the artist was or the style they chose or even if I liked the piece or not, the one thing that every artwork possessed was an arrangement of shapes. They could be very simple or very complex, but there was always a variety of shapes in particular colors with a variety of values.
It is also called form. In fact, the definition of form is....a shape; an arrangement of parts. This doesn't seem very important until you see a painting that simply has a monochromatic background with splotches of color all over. If those splotches are all very close in value to the base color, that would be a very weak painting, if you could call it that at all. Even colorists have used an analogous color scheme where the colors are extremely blended as they go from one to the other have created form within the painting.
I have this on my mind because it is very good to frequently go back to the basics and figure out these things. So shapes become something that require very careful attention. In addition to having interesting shapes that have contrast of size, it is good to then make sure the edges of those shapes have a variety of soft and hard edges. If there is too much of one or the other, (especially all hard edges), the painting would look very contrived and immature.
So to walk through a gallery and pay particular attention to shapes and edges is a very instructive thing to do. It will serve as a reminder of what to do or not to do in your own work. It is part of learning to analyze design in order to know how to do it yourself. And there you have it...just a few more things to think about.