This "diet" will have you strive to do every single thing to make you healthier, within reason. It's not about losing body fat but about becoming as healthy as you can be and losing body fat as a consequence. It isn't even necessarily low carb (well it's lower than 150g for the most part, but that's hard to do without refined carbs), it depends on the person. Read the blog and Mark's book and compare them to Atkins. There are some similarities which happens the be the case when two people stumble upon a good idea, but the approaches are totally different. Mark isn't only about diet either. He will do anything at all to become healthier. Sleep, exercise, lifestyle, etc.
Diet-wise primal is paleo with optional dairy products and more fat. And Cordain seems to be down on nightshade vegetables where Mark sees it as more ambiguous.
Stabbing conventional wisdom in its face.
Anyone who wants to talk nutrition should PM me!