Lots of great information on this thread. I'll add my thought and try not to cause a war!
Actually, foods we crave are genetically programmed into us, they are NOT bad foods, in fact, they are necessary for survival. In the past, starvation was the problem, not being too fat. This is true of all animals, not just humans. Only humans who are living with food abundance believe fats and carbs are bad, they actually are good for you, but we need other nutrients as well. Corn got a bad name because mixed with other amino acids it forms a protein, and some companies were listing a high percentage of protein in their food, and they were allowed to count corn with the overall percentage of protein. This is misleading if you haven't studied nutrition, but it's not such a bad thing unless corn is listed as the first ingredient in a food. So try to avoid foods where the first ingredient listed is corn. Some people say their dogs developed allergies to corn, but most dogs have no problem with it. This site has some good information about nutrition, and the author has great credentials.
The Dog Food Project - Myths about Dog Nutrition