I think there some misunderstandings are derived from terms like 'a vegan diet' or a 'non-vegan diet', because there are no such thing as one vegan diet or one non-vegan diet.
Some people go vegan in a situation where they don't feel happy with their health or what they eat. I'll use B12 as an example (surprise! ). These people may come from a background of both eating a lot of meat and dairy products AND supplements (circa 50% of non-vegans take some kind of food supplementation). They may have very high B12 levels, which are associated with a number of health issuesOr: they may come from a background of not eating enough proper food, possibly combined with a high intake of sugar, coffee, tobacco, maybe alcohol - and have very low B12 levels before they go vegan. If they are B12 deficient before they go vegan, normal B12 supplementation may not do the trick for them - they need 'medical amounts' of B12.
A traditional, non-vegan diet is normally lower than a balanced vegan diet in a lot of nutrients, like folate, vitamin C, phytochemicals etc. According to USDA, the average American diet is deficient in calcium, iodine, vitamin C, vitamin E, fiber, folate, and magnesium (40% of non.vegans also have low B12 levels according to several studies). According to this information, a person jumping straight from a traditional American diet to eating vegan food, should pay a little attention to - and maybe try to find out - what his nutritional levels are before going vegan, and try to adjust and improve his levels by eating food that contain healthy amounts of whatever he is missing.
Since animals eaten by meat eaters normally live on a lot of supplements, new vegans should also pay attention to what supplements he won't get anymore because he stops eating them via eg. calcium fortified milk or meat from cow's grassing on cobalt-fortified soil. There are some areas in the world where the soil is poor in certain nutrients, and these nutrients are therefore added directly or indirectly to whatever factory animals in these areas eat. Since both vegans and non-vegans today often eat a lot of food from other areas than where they live, they may get eg. selenium in their diet even if the local soil is selenium deficient. If not, one can find out what plants that contain selenium (eg. Brazil nuts) and eat a little more of these, which IMO is a more reliable solution than eating pills.
I see it in a slightly different way: there are some nutrients non-vegans need to pay extra attention to, and there are some nutrients vegans need to pay extra attention to. The calcium in cow's milk comes from plants the cows have been eating. Meat eating animals are almost exclusively eating - plant eaters, who have gotten their nutrients from plants!There are vitamins and minerals that humans get from meat and dairy, and are rare or humanly indigestible from plant sources.
In general, I'd be more worried if a person who were living on a balanced plant based diet would decide to go back to eat traditional, non-vegan food.
I'm stressing this because meat eaters and new vegans some times have a tendency of believing that vegans have to pay attention to what they eat, while non-vegans don't: this is a very common myth. Again, to use the good old B12-molecule as an example: vegans need to be more worried about B12 deficiency than non-vegans, because while members of both groups may be equally exposed to a number of B12 killers, meat eaters consume more B12 than vegans, and since both groups live in a very B12-unfriendly world, meat eaters are less likely to become deficient.
I'd rather say that this is a list of nutrients that some vegans may need to pay attention to. The vegan diet as such doesn't eg. lack zinc: according to vegansociety.com, 'diets of vegan and non-vegan children often contain similar amounts of zinc, though zinc from plant foods is less well absorbed as they contain phytate, which interferes with zinc absorption'. Nevertheless, there may be vegans who have low zinc levels (if they don't eat plants containing zinc: Look here. Or here. Or here... ) (Zinc supplements may be needed for young vegan children whose diet is based on high-phytate cereals and legumes, again - according to The Vegan Society.The vitamins (and EFA's) most commonly lacking in a vegan diet are: