I don't mean to pick on you but let's go through this so everyone understands.

Actually, I heard it was the other way around - high protein causing kidney problems. It was a big argument against raw feeding way back when.
There was a study decades ago where they made rats ingest massive amounts of protein. The rats did not fair well so the conclusion was made that high protein is bad. They did not consider that rats are not carnivores. Dogs have the ability to utilize large amounts of protein.

Flaxseedoil1000, you raise a valid point and while GOOD by-products are certainly benefitial and essential in a dog's diet, an ingredient's list does not do a food justice, or lack thereof. You don't know whether by-product means livers and hearts or beaks and feet (btw, I know poultry feet are a popular raw food item). You don't know the quality. You also don't know the AMOUNT of a said ingredient in kibble. You get a general idea of which ingredient dominates the kibble, but you don't know exactly how much of said ingredient is in it. An ingredients list is so vague and generalized, it is difficult to judge a kibble based simply on the ingredients list so much of this is up for debate.
True. I suspect, but do not know as fact, that the low end dog food producers (which is the vast majority) lobby to keep the information on the bags as vague as possible.

Wouldn't it be nice if instead of them listing ingredients in order by weight they showed the actual percentages? I'm not holding my breath for that.

There was a time when it was not allowed by law to show a picture of the actual kibble on the bag. Now the rules have just changed and they can start listing the calories per serving on the bags.

However, as a general rule, dogs are carnivores. Therefore, it is unnecessary to include carbohydrates into a dog's diet (both simple and complex), i.e. corn. As well, while carbohydrates are a worthy source of quick and efficient energy, lipids and proteins would do a carnivorous dog much better.
Yes, dogs are not true carnivores in the sense that cats are. When carbs are present they will be used for energy leaving proteins and fats for other uses.

Also keep in mind different carb sources are processed more quickly than others. For example rice is like drinking a can of Red Bull. Glucose levels go through the roof. Great if you have a Grey Hound that needs to run a race. Not so good for the average dog and definately something to avoid of you have a diabetic.

You also mentioned that plant based proteins are troublesome and do not provide a viable source of protein. Therefore, the rest is thrown out as waste. Does that not sound like filler to you?
It depends on what your definition of "is" is. Oops, wrong topic. It depends on what your definition of "filler" is. To me a filler is something that has little or no nutritional value (good or bad), it is just there to take up space, to "fill" the dog up.

That is not the case with corn. Also keep in mind there are many components in corn, that's why you see it listed as corn, corn meal, corn glutten, etc..

Corn is about 8% protein (incomplete in essential amino acids), 62% carbs and the rest is fiber, oils, ash, etc..

Corn glutten is the protein part of corn. Other parts of the kernel have been processed out and the result is about 20% protein for glutten. This is a red flag for me as they can use it to boost their protein level in the guaranteed analysis. There are actually a couple of types of glutten, one is fairly "digestible" (for cows anyway), the other is not. Either way corn glutten is something I avoid.

Corn meal is just about the opposite. The glutten and fiber have been processed out, leaving mostly carbs. This I have no problem with. It's good source of energy and it is not messing with the amino acids.

While a certain amount of carbohydrates may be acceptable and even beneficial to a dog's diet, they should NOT make up the bulk of the diet. This is why I must agree with MyPeanuts. Corn as the number 1 ingredient is something to avoid and I do believe it is considered a filler when it is used so blatantly to fill up space.
Agreed. I prefer kibble high in protein and fat. I have been feeding 30% protein, 20% fat for sometime now. I would not do this with a dog that needs to lose lots of weight, but then I never find fat strays.

There are many variables that may affect the quality such as the cooking temperatures/times, quality of ingredients, etc. However, I truly believe that corn (and all parts of corn) and byproducts as the first ingredients is a signal that the company is trying to fill the kibble up with cheap ingredients. This is just my opinion. The Food Debate was named so for good reason.
I understand where you are coming from.

I do however take exception when I see/hear "by-products are bad" and "corn is just filler".

There is much more to it than that and folks need to educate themselves.