How do you describe what a good grocery display should look like?

Depends on exactly what type of display you are talking about. You're basic end cap display should include 2 different items preferably. The bottom portion, or "base," should be the largest section of the display. Then a shelf of something that ties in with the bottom. This is the "shadow" shelf. Lastly, the top portion should duplicate the bottom. The top should be smaller than the base. Roughly 2/3 as big. A typical end cap might include laundry detergent on the top and bottom with fabric softener sheets in the middle as the shadow.