Ok, I will start this by saying that I sell Iams Pet Food. I also sell several other brands mentioned in others posts including Blue, Nature's Variety, Nutro plus about a dozen other brands.
I have had hours of training in dog/cat nutrition. I spent 2 solid days in training for a company just last week. I just have a couple of things that I would like to point out.
1. There is no ONE dog food that is right for every dog.
2. Iams removed ethoxyquin from their diets at least 8 year ago (sorry don't remember exactly.
3. Corn as a carb is good. It produces less glucose spikes. Think of how you feel if you eat a candy bar. Great in the beginning but then you need a nap. Corn helps reduce those highs and lows.
4. Corn as a protein (corn gluten) isn't very good.
5. NO long term research has been done on the affects of feeding a grain free diet.
6. Yes, animal protein should be the number one ingredient in a food but watch out for breaking up of ingredients. rice, rice flower, rice bran all in one food would out weigh the original animal protein.
7. Less than 15% off allergies in dog are food related. Unless you have had testing done you will never know if something didn't agree with the pet or if they are actually allergic to it.
You're totally lost me on this, particularly #3. Human's don't even digest corn, so why should we believe that carnivores do? What is the value of putting a cheap (and highly subsidized in this country!) filler in the food instead of something that can actually be digested and used?
Also, I don't care how much research has been done on grain free food.... how much research has been done on cheap crap kibble and the long-term effects? Commercial kibble has only been around for a few decades, and before that people were feeding their dogs on meat and kitchen scraps. And you can believe that they weren't feeding their dogs primarily corn. Also, most low quality (ie. IAMS) kibble is full of chemicals - the rate of canine cancer is insanely high (as high as 50% in some breeds) right now, and we have to believe that it is coming from somewhere. My high-quality grain free kibble is as close as I can get to a balanced diet without cooking for my dogs myself.
And if only 15% of allergies are food related, that still means that a significant number of dogs are going to have problems with certain ingredients. One of my GSD's cannot tolerate corn, soy, beet pulp, or wheat gluten. He also does badly on lamb and poultry. That rules out all of the low quality foods, and many of the higher quality ones as well. My other GSD, on the other hand, will have a gorgeous coat and perfect poop, no matter what she eats. Different dogs = different needs.
I am glad to hear that IAMS has phased out ethoxyquin, though. The sheltie that I grew up with ate IAMS his entire life, and had trouble with skin allergies, joint problems, and eventually died of cancer earlier than he should have. I am not entirely convinced that the food was not at least in part to blame.
Now if he'll just eat when we put the food in his bowl. 
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