In each piece there needs to be a clearly defined, or at least an accessible aura of nature-based reality.
Things are either heavy and they fall, or they are lighter than air and they float. An object is either thrown from one place to another or it is capable of propelling itself. Either it is solid, liquid or gas. There are no real exceptions to the last one—well, plasma, I suppose. Ecto-plasma, anyone?
Either it is a face or it is a cloud. Or maybe it’s a little bit of both.
The point is, a visual or concrete poem (that shows an object through words rather than looks at words particularly) should look like what it is or what it describes. If the poem is about a circle, it shouldn’t look like a square.
Unless you’re sarcastic, ironic or surreal. In which cases, you’re on your own.