Good consistent design and choice of colors. In itself an accomplished harmonious piece of art that would hang nicely as a painting on a wall, too.
Only this is computer art. Now depending on your goal say in connection with a wallpaper for a computer screen, you may want to extend your image to match one of the screen formats in use, e.g. 1280x1024 or anything where the height is more than the length of one of your image's sides. That way you fill the entire backgroud of a standard computer screen with something matching rather than leaving it up to chance.