In regard to complete protein and vegan diets (a vegan being someone who doesn't supplement their plant based diet with dairy products or eggs), it is important to note that a vitamin B12 supplement would be necessary. B12 only occurs in animal based products. A deficiency would take a long time to show up, and by the time it did, severe nerve damage could be a problem. A lacto-ovo vegetarian wouldn't have this problem (lacto meaning milk, ovo meaning egg). A red, swollen, sore tongue is a cardinal sign of a B12 deficiency.
All vegetarians need to be aware that heme-iron (iron fom meat) is 30% more absorbable than non-heme iron (iron from plants). Therefore, it would be important to eat foods high in vitamin C to compliment your non-heme iron, because vitamin C makes non-heme iron 30% more absorbable. So eat strawberries with your spinach, or drink unsweetened grapefruit juice with your beans. Beans and spinach are two plant-based foods rich in iron.