I am interested in several of the usual visual things: form, texture (which I think could be considered line—lots of them), tone (actually value), last and least is color.
Composition used to be a much bigger concern for me than it is now, though that might be to the detriment of my images. But I think I have simply decided that the lack of a powerful composition is not in itself a show stopper. Sometimes I deliberately defuse a composition to see if I can make an image work on the strength of other things. But I think there is always composition.
Composition is the choice we make, consciously or otherwise, when we frame an image. The rectangle of the viewfinder or ground glass imposes itself and its aspect ratio on everything viewed, quite unlike human vision where the shapeless periphery gradually fades away. Everything that happens in a photograph, a painting, or a print, happens in direct relation to the frame. It is that interaction that causes the compositional tension and movement we sense in an image. Could negative space exist if a form didn't have the frame to play off of? Actually yes because it can be created by the interaction of two internal shapes. Regardless, from a strictly formal perspective the frame is the context for everything in the image and defines formal content—strong or weak, recognized or not.
Texture has gained in importance for me. I have developed several tools for emphasizing textures of different frequencies in an image. I organize masses of texture into a compositional whole, which makes landscape photography here in the east (where things tend to be close together and lack monolithic structure) possible for me. If one insists on composing with complete objects of significant stature one will have trouble. Everything is too close together and obstructed for that. One has to learn to see differently here.
Tone is a bit more difficult to talk about. I retain the love of tonal relationships of black and white photography. There is something lovely, pure, and quietly elegant about properly placed tonal values in a good monochrome image. And fortunately it works in well crafted pigment prints (inkjet) just as in silver or platinum / palladium prints.