The overall concept of the Paleo Diet is to duplicate the metabolic milieu that we evolved under, while also staying consistent with our individual needs . It does not include duplicating everything about the way our Paleolithic ancestors lived. There is no reason to reject technology and return to the stone age or to become hunter-gatherers. The core concept also leaves plenty of room for person-to-person differences for things like allergies, individual taste, etc.
Duplicating the Paleolithic metabolic milieu is the reason for eliminating grains, since they weren't a big factor in the diet of humans until the advent of agriculture only 10,000 years ago. Grains are associated with gluten sensitivity, which is much more widespread than commonly thought. This approach also leads to:
An important and related dietary guideline is minimizing insulin levels. This is because insulin is known to be a pro-aging hormone, and is responsible for a large constellation of modern health problems, ranging from high blood pressure to heart disease to diabetes and even cancer. Although it partly follows from the concept above, it needs to be called out on its own, since several derivative principles aren't clear otherwise, such as minimizing carb intake and adding support to the benefits of exercise. How strict one needs to be to achieve this goal can vary tremendously from one person to another. You can get some idea how you're doing by measuring your blood glucose level an hour or so after you eat (using an inexpensive meter), and by using KetoStix to watch for ketones in your urine (small amounts of ketones means you're in mild ketosis, which also tends to imply that your insulin levels are low).
My personal spin on Paleo includes minimizing toxins: first in my food and drinks, and also with things that I smell or put on my skin. Through research and personal experience, I've learned about the tremendous negative impact that even micro amounts of toxins can have on the body (the brain and the immune system are especially sensitive), particularly over long periods or if you're starting from a nutritionally deficient state.