Protein - Essential for muscle development, and found plentifully, in its complete form in foods such as beef.
Vitamin A - Although humans can synthesize vitamin A from beta-carotene, the process is ridiculously slow and inefficient. It is much easier to get it directly from animal sources. Herbivores are much better at synthesizing it, hence their ability to use beta-carotene as their only source of vitamin A.
Vitamin B12 - Essential, and naturally found only in animal products. Herbivores synthesize it themselves, but we don't, so we need an animal source.
Iron - Meat contains large amounts of heme-iron (the kind which is easily absorbed). Plant iron is almost entirely non-heme, and is more likely to damage the digestive system.
Vitamin C - It has been recently discovered that liver contains enough vitamin C to sustain you! The vitamin C requirement is very low in humans as opposed to other primates, making it possible to survive on animal products alone.
Cholesterol - Forget
what the quacks at the American Heart Association have been telling you! Cholesterol does
not clog arteries. In fact, cholesterol is very important to human survival. You can not live without it.