A valid criticism is that there are inconsistencies in the tenets of the paleo diet, especially when it's practiced dogmatically. I focus on keeping junk fats, junk carbs, processed foods, legumes, gluten and casein out of my diet, I don't get dogmatic, I don't believe that there is one rigid right way to do Paleo and I'm good.

Here are some inconsistencies pointed out in one review of Paleo.

If the argument is that because "some" lectins are toxic to "some" people, then "all" people should avoid "all" lectins, we have a problem. We live in a world where food exists as part of a chain, with predators eating prey – and the prey develops defenses to protect itself from being eaten. Lectins are part of the circle of life and can’t be avoided; they permeate the food chain as predator eats prey. This means that if you wish to avoid all lectins, you would have to avoid all food, since all food contains lectins. To do otherwise implies selective belief in your theory. At least the Blood Type Diet acknowledges this issue and says that at least some groups of people have adapted to eating grains, beans, potatoes, and dairy.

And, another:

Fruit and root vegetables such as carrots, turnips, and beets are okay, but not tubers such as potatoes, sweet potatoes, and yams. Incidentally, I find the exclusion of tubers requires a bit of theoretical bending. The argument is that potatoes are a "new world" crop and humans have only been eating them for maybe the last 35,000 years. But in truth, yams are an African crop that people have been eating since the dawn of time. So why are they excluded? And if that's your logic for excluding potatoes, then why is turkey okay? After all, turkey is a "new world" species, not even introduced into Europe until the 16th Century.

And, this:

As for fruits, berries of all kinds are good -- strawberries, blueberries, and raspberries etc. are good. From there, differences in Paleo's abound. Tree fruits are controversial. For example, some say apples are great. Others call them "bags of sugar." And still others say they're okay if you eat the low sugar varieties. And yet, if the theory is based on eating what hunter-gathers ate, then tree fruits would have to be top of the charts. Not to go Biblical, but I think it's pretty safe to say that tree fruits such as apples and pomegranates have been part of the human diet since the very first man and woman walked the earth. And I don't believe hunter gatherers selected their fruit based on the glycemic index.

And, more on lectin:

Also, fruits contain lectins3 -- just like grains. Apricots, bananas, cherries, kiwis, melons, papayas, peaches, pineapples, plums, and even berries are all known to contain lectins and cause allergies. In fact, fruit allergies make up about 10 percent of all food related allergies. So why are fruits allowed? Incidentally, new research has shown that allergies to fruit are actually made possible by pectin, the soluble fiber found in fruit. The pectin surrounds the fruit allergens in the digestive tract so that they don't get broken down and enter the bloodstream intact. Using a digestive enzyme supplement that contains added pectinase can help moderate that problem by breaking down the fruit pectin, which then exposes the allergens to digestive juices and enzymes.

And, another inconsistency:

The assumed diet of the hunter-gatherers modeled by the Paleo's is reflective of cave people living in Northern Europe in cold climes where plants did not readily grow. But the simple truth is that hunter-gatherer societies in other locations ate decidedly different diets. As Katharine Milton points out in an editorial in the American Journal of Clinical Nutrition: "The !Kung might live in conditions close to the "ideal" hunting and gathering environment. What do the !Kung eat? Animal foods are estimated to contribute 33% and plant foods 67% of their daily energy intakes. Fifty percent (by wt) of their plant-based diet comes from the mongongo nut, which is available throughout the year in massive quantities. Similarly, the hunter-gatherer Hadza of Tanzania consume "the bulk of their diet" as wild plants, although they live in an area with an exceptional abundance of game animals and refer to themselves as hunters."

And, on grains:

And it's not just modern examples of hunter-gatherer tribes. There is solid evidence that suggests that Paleolithic peoples commonly ate grain, and even flour, as far back as 30,000 years ago.5 In fact, there is quite reasonable evidence that people were processing cereal grains for food as much as 200,000 years ago.6 The bottom line is that the fundamental premise that Paleolithic peoples did not eat grains and that they ate large amounts of meat is only "suggested" by historical records, not necessarily supported by them.

And, one more:

The idea that the so-called Paleo Diet is inherently healthier is simply not supported by the evidence, either ancient or modern. What is supported is that eating modern highly processed, high-glycemic foods is unhealthy. Diabetes was virtually unknown in China until people began eating the modern Western diet. But before people started eating modern diets in China, they weren't eating anything remotely close to the Paleo Diet. They were eating a largely vegetarian diet grounded in rice and noodles. For centuries, they ate grains without problems. It was the introduction of refined sugars and oils and processed fast foods "what done em in," to quote Eliza Doolittle. As a side note, although meat consumption has gone up dramatically in China, with disease rates climbing right alongside them, it's probably not the meat that's causing the problem. It's most likely all of the refined, processed, fast food that's killing them.

It's environmentally difficult to provide a mostly meat diet to the majority of humans. But this is not really a criticism of the diet. It's more a criticism of humans for overpopulating the planet.

I think that because it's so contrary to the food that's readily available to us, it requires a large amount of planning and preparation that can easily slide into obsession and orthorexia. And because it requires us to give up a lot of socially enjoyed foods, it can promote social isolation. As I come from a background of eating disorders, these are my chief concerns - in many ways it often feels like an eating disorder.

It can be expensive compared to the subsidized food in the SAD. That is a valid criticism. (The benefits are worth the price.)

Many people derive a lot of joy in life from eating things like pizza, donuts, ice cream, apple pie, cake, chips, snickers bars, etc. Eating paleo deprives you of this joy and for many perhaps the health benefits do not outweigh this deprivation in terms of net happiness over your lifetime.

This is one I struggle with at times. I think it's likely a "grass is always greener" situation where I'm envious seeing others enjoy engineered foods, but if I put myself back in their place, I wouldn't end up happier. Hard to say for sure, though.

I think the biggest problem with paleo is that it's quite hard to do it in our current society. The diet itself doesn't have many problems, except maybe the tendency to eliminate types of food that could really improve the quality (nutrient-density) of the diet (eg. dairy).

In itself the diet is pretty good as it addresses the 2 biggest problems in the SAD : PUFAs and grains (= phosphorus, anti-nutrients and gluten).

Other criticisms often involve 'tendencies'. Lots of paleo people take quite some supplements, have a very low calcium intake, eat plenty of PUFAs (nuts), have a very high iron intake, a high tryptophan intake, .... Telling people you don't eat grains because Grok didn't eat them while popping pills and eating loads of muscle meat (basically a refined food) can be confusing to others. But all these things aren't part of the paleo mindset but rather choices/mistakes that people make.

It is more expensive and more difficult than the SAD, but these things are debatable : the acquired increase in energy and improved mental state (hopefully) likely makes it easier to work more and earn more money.

Some won't like the high animal product intake, I think that's silly.

That humans have not adapted since paleo times. Every other species - from goats to cattle to wheat to bacteria - adapts within several generations. Lactose tolerance and skin pigmentation argue the same for humans, yet paleo pundits argue that we can go backwards on the evolutionary timeline.

Not necessarily a criticism of the diet but of the psychology of the people who follow it; which is the same psychology of people that follow anything like a Vegan Diet or the Next BIG network marketing deal or the next big workout deal,etc. I would like to echo the thoughts of Danny Roddy on his farewell podcast of the Healthy Skeptic. We are all such different beings yet we are so dogmatic about what someone else should do. Just because it worked for you does mean it will work exactly the same for everyone. It is easy to find something that really benefited you and want to share it but then become very close minded to the fact that there may be other ways to solve the problem. Hey if you have a headache and drinking tons of water on a low histamine diet, taurine diet and still not getting relief then maybe you should try an aspirin. All the infighting regarding low carb or low fat or low this seems silly to me. It all potentially works, why the big ruckus. We shouldn't look at this as a cure all as many do (and then are deeply disappointed when we are not cured) but rather as another tool to manage your situation.

EDIT: Now I'm on a soap box. I would also add that I don't understand how the paleo community can be so anti this or that and be so pro alcohol. There are so many studies that show that alcohol negatively affects our bodies. I watched a documentary years ago that pointed out that alcohol (unlike other drugs) affects every part of the body. They compared a group of heavy heroin user to a group of severe alcoholics and after 40 years the heroin addicts were alive but everyone in the alcohol group was dead. We seem to want to invite data that supports our beliefs but almost completely ignore data that doesn't. Okay now I'm off my box.

The main criticism is that it hasn't been studied thoroughly. Whereas you have decades of SAD research (admittedly, most of it biased and not taking into account other lifestyle factors such as smoking, alcohol intake and genetic predisposition). But seeing some controlled studies done on Paleo/Primal, and having the results published in medical & nutrition magazines/sites would go a long way towards legitimizing this way of eating.

tl;dr Paleo hasn't been vetted thoroughly.

Valid criticism of the "paleo diet" as followed by many, or the way out anscestors/hunter-gatherers ate/eat?

If you mean the generic term, there are probably lots of individual points of eating habits that could be criticised but it depends on the diet itself.

However, re hunter-gatherers etc -Two I can think of.

Animal welfare. This is not to say veganism is 100% valid or consistant either, but animal welfare is a reasonable consideration - preferably animals should lead good lives before we eat them.

And practical/economic.

One basic criticism is that grains are the staple of so many healthy populations. Following,

why would one be better off wholly eliminating them?

This is an old and basic criticism but I continue to think it's valid. Populations thriving on rice diets will laugh at the thought.

WAPF, GAPS and SCD are not under the Paleo umbrella.

Most centenarians eat little meat and lots of legumes. Dan Buettner's team researched this.

(BTW, search for "Paleo" in Buettner's Ted talk page).

Then again, the first sentence of this answer seems to only be a criticism of certain Paleo diets. To copy a comment on http://www.outlawfitnesshq.com/the-paleo-diet/

"There is a book out there from before the current paleo craze, “The Paleolithic Prescription,” published in 1988 by several authors including Melvin Konner (an M.D. and anthropologist) and Marjorie Shostak (anthropologist). The two of them lived with and studied the !Kung people of the Kalahari. Their book discusses the hunter-gatherer pattern not just for diet but for childrearing, exercise, etc. Their advice for incorporating a ‘paleo’ take on nutrition for current-day humans? 60% carbs, 20% protein, 20% fat, “Carbohydrate should be mainly complex, with little simple sugar and abundant dietary fiber. Protein should come primarily from animal sources which in our society means poultry, seafood, and low-fat dairy products: vegetarian protein-balancing is an acceptable alternative.” Their suggested diet also includes whole grains and legumes. So even those who think our current diet should follow a hunter-gatherer model have come to very different conclusions about what that means."

See the above link for more seemingly valid criticisms.

http://www.chicagotribune.com/health/sc-health-0420-palos-diet-20110420,0,7758445.story also has criticisms of cutting out legumes and grains.

That its too expensive. True for some. A lack of defined macronutrient ratios. You could eat raw honey and sugary fruits all day and still be "paleo" That's the only valid arguments, really. Everything else about paleo is pretty awesome! I for one am feeling and looking (IMO) better than ever!