Good question. Long body is not the look you want. "Football" is another term used to describe lower quality fish.
Discus, as the name implies, should be round. So the outer edge of the fish, including the dorsal and ventral finnage, should be circular. In some newer strains, just the body alone is round. With the finnage, these fish are actually taller than longer. I happen to like these more, but it's obviously a personal preference. Ironically, not all wild caught discus are round.
Discus shape is controlled by both genetics and environment. In my experience, discus fry will get rounder taller with lots of water changes and heavy protein diets. You can see this effect in some of the postings on this site.
Hope that helps.