Quick recap: For optimum health and performance, eat a paleo diet - a diet composed entirely (or at least predominantly) of foods available to our ancestors during the paleolithic era. That means meat (preferably from animals also fed a paleolithic diet, which means grass fed beef), vegetables, fruit, nuts, eggs, and berries. That means especially no grains (wheat, barley, oats, corn, rice), legumes, or dairy. Different versions of the diet tolerate breaking this rule to a greater or lesser extent - some permit certain types of dairy, some corn or rice (but rarely wheat), and so forth. Nobody owns the concept, so you'll see "paleo" authors and bloggers differ, but the emphasis on meat and veggies over grains and sugar is pretty constant.
Having established that, I think that people following a paleo diet go wrong in two different ways, to greater or lesser detriment. The first is not recognizing that not all neolithic foods are bad for you. The second is not recognizing that not all paleolithic foods are good for you.
The first point is less of a serious problem than an inconvenience for the eater. If you take the "paleo" concept too religiously you will avoid foods that are probably okay to eat and might even be good for you. Many paleo authors openly endorse a handful of neolithic foods, based on whether the foods in question contain anti-nutrients or toxins that cause problems. Chocolate is a good example. While the sugar and milk solids that go into commercial chocolate are definitely problematic, cacao itself doesn't seem to have negative effects on health. So very, very dark chocolate is probably okay, at least as an occasional treat - and it's certainly a pleasant break from meat and veggies at every meal. Butter, especially clarified butter or ghee, and cream are also relatively unproblematic - they don't contain the milk proteins or lactose that cause digestive problems. I suspect tea and coffee also belong to this group, although the evidence on the safety of limited amounts of caffeine is somewhat mixed. Another (little discussed) example of an absolutely neolithic food that really has no health impact at all is seltzer - it's just carbonated water. No impact on digestion or the immune system. Also definitely something cavemen didn't drink. Now if someone else wants to avoid seltzer because it "isn't paleo," that's fine with me, and it's no detriment to them, of course, as long as they're not too bored drinking plain water.
As far as the second point goes, this is where some people get themselves into trouble. If you look around the paleo websites, especially the cooking oriented ones, you can find recipes made from ostensibly paleo ingredients that are probably serious problems. I've seen desserts made with crusts made of nuts, fillings made from dried fruit or crushed figs or nut butters, and more nuts on top. I'm sure we've all been tempted by bags of dried fruit or nuts that seem paleo.
The problem isn't that nuts or fruit are absolutely bad for you - the problem is quantity. If you make a pie out of nuts and fruit with an almond butter/cocoa topping, and have a slice for your birthday or Christmas, you'll be fine, and you'll be better off than if you ate half a dozen regular cookies or a big piece of regular cake. BUT if you make that pie and have a slice each day after dinner and two slices on weekends, you're regularly getting a huge load of omega-6 fats (and not omega 3's) and fructose, both of which can lead to insulin resistance and increased inflammation. You could probably make yourself quite sick and fat on a paleo diet, if you tried. Now if you eat a handful of nuts each day and an apple or half a cantaloupe after dinner, that's fine. But if you chow down on a bag of dried fruit or keep an open bag of almonds by your side all day long you can eat yourself into some real problems.
The take home message:
You can probably make yourself fat and unhealthy by eating too much of almost anything, but within reason there are certain foods that can make up the bulk of your diet without causing problems, foods that are okay to eat daily, foods that are okay only as an occasional treat, and foods that you should probably never, or very rarely, ingest. You need to keep a handle on which is which: I group foods into staples (make up the bulk of my diet); daily but limited (I'll eat them all the time but in smallish quantities); treats (I'll eat on the weekend or for a cheat day or if I'm out to dinner); and forbidden (self explanatory).
My staples include grass fed beef, coconut oil or animal fat, green leafy veggies, and green tea. My daily but limited foods are 85% or higher dark chocolate (no more than 100 cal/day), root vegetables (1 lb/day), caffeine, nuts and fruit. My treats are grain fed beef, dried fruit, ice cream (less frequently), white potatoes, food made with vegetable oils (like Taro chips), and 60%-85% dark chocolate. My "forbiddens" are basically wheat, diet sodas, and legumes.
The categories aren't important - what does matter is that you organize your foods in a way like this. I find that if I try to forbid myself all but the healthiest foods I'm asking for trouble. By having a treat and limited category I give myself permission to "cheat" or indulge myself without eating the most harmful foods. But I remember not to make these foods part of my every day menu.
Exactly where should each possible food item go? Here things can get tricky. I can only suggest that you both continue reading and continue experimenting on yourself. Are you feeling lousy? Having trouble sleeping? Look at what foods may have crept into your daily eating that are problematic. Six months ago I tried to keep all starch on my "treat" list. It didn't work out for me - specifically when working out with high intensity. So I moved things around.
Remember that the paleo idea is just a guideline. There is no logic to the notion that every neolithic food is bad - but there is a lot of logic to the idea that we should be much more skeptical of the healthfulness of neolithic foods and much less skeptical of the healthfulness of paleolithic foods.