Most dog foods contain lots of animal by-products, referred to as "meal", ie chicken meal. Basically, it's the leftovers, like the beaks and bones (yeah, yuck). Also, most contain lots of wheat gluten. All of that is tough for a dog to digest.
Look for natural foods, such as California Naturals, Wellness 5, Propack, Nutro Natural Choice, or Canidae (I use this one). They all have REAL meat - no by-products. The bag should advertise that. Canidae is the best for staying away from high amounts of wheat gluten. All that is is filler. Many dogs are allergic to wheat flour.
There are also vegetarian dog foods out there. I've not tried any yet but am considering it. Let me know if you find a good one.