Probably B12. It's sufficiently abundant in dairy and eggs that run-of-the-mill vegies don't have the serious problems vegans do. It must be supplemented and I've read diverging opinions on the efficacy of plant-derived analogs, synthetic B12, and animal derived. The literature is pretty firm that B12 from animal sources are more easily absorbed by the body.
AL Rauma et al. Vitamin B-12 status of long-term adherents of a strict uncooked vegan diet ("living food diet") is compromised. J Nutr, 1995, 125:2511-5; found that most if not all (raw food) vegans have severe B12 problems.
MG Crane and others. Vitamin B12 studies in total vegetarians (vegans). J Nutr Med, 1994, 4:419-30; found the same as Rauma.
I Chanarin and others. Megaloblastic anaemia in a vegetarian Hindu community. Lancet, 1985, Nov 2:1168-72; This is an interesting one. It seems that some of the Hindu vegans living in India do not have B12 deficiencies. The study looked at one grop that relocated to England and, while their diet stayed pretty much the same, they quickly developed anemia- a complication of B12 deficiency. The researchers identified insect eggs and larva in the Indian food supply as the source of the B12. I'm sure the Hindus were just thrilled to find that out.
Interestingly, there's a school of vegetarian thought that levels a similar claim at meat eaters. That the terror an animal experiences at the time of death is imbued into the meat and transfered to the person that consumes it causing that person to become irrational and violent. "Dr." Ballentine (sp?) I think. Haven't found anything in the literature to support it.
One of the things that interests me is that any time radical vegetarianism or vegans are discussed because of their attempts to "prove" their way is best and the other way will lead to doom, a lot of meat eaters respond in exactly the same manner, citing so-called studies that spout one theory over another "proving" that those who do not eat meat are destined to be deficient in one health area or another.
Why can't we all just get along and eat what we want and suffer the consequences for our decision? I don't eat what I don't want to eat, i.e., the flesh or body of anything with fur, feathers, blood, veins, or anything that shits in the same manner as a human. It's just that simple. I don't eat what makes me sick to my stomach, a list which is not confined solely to meat. I don't like nuts in my brownies, pineapple on my pizza, broccoli in my "cheese" casserole or coconut on ANYTHING.
On the other hand, I could not live without cheese and will not avoid sweet potato pie simply because it has eggs in it. My people are farmers: cattle, pork, corn and soybeans. I prefer the corn and soybeans but, as stated before, I don't care what anyone else is eating as long as they're not on the Dahmer diet. And that includes dogs, cats, and pigeons too. People should be free to eat any non-human thing they want to eat free from the condemnation of others.
BTW, yours was an interesting post and confirms the rightness of my inclination to swat the fly while it sits in the potato salad, not after it's flown away.