Meat, fish, eggs and dairy products are common sources of protein for most people. Americans who consume these foods typically get plenty of protein in their daily diets, but vegans need to turn to beans and nuts to get enough. 
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Possible Iron Deficiency

Meat, particularly beef, and shellfish are rich sources of iron for omnivores, but it's important for vegans to consume iron-rich foods as well. Iron deficiency can lead to fatigue and problems with brain function.
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Getting Enough Calcium, Vitamin D and Vitamin B-12

Vegans need to eat plant sources of calcium, such as dark leafy greens or fortified soy products, to get the recommended 1,000 milligrams per day. Vitamin D helps the body absorb calcium. Vegans need to either take a vitamin D supplement or drink soy milk fortified with vitamin D. 
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Difficulty Eating Away From Home

A July 2012 Gallup poll reported that only 2 percent of Americans identified themselves as vegans. This means it's sometimes difficult to stick to a strict vegan diet since most of the country caters to those who eat animal products. It's often hard for vegans to eat out since many restaurants don't offer many vegan choices. Parties and family events can also be difficult, though vegans can minimize this challenge by bringing their own meals if they know there won't be any choices that conform to their dietary restrictions.

Breeding, raising, and feeding animals for food is a tremendously inefficient use of our natural resources. Animals raised for food production are fed over half of the all the world’s crops. As our population grows, we require more and more agricultural space. 60% of worldwide deforestation results from land being converted for use as agricultural land, much of which is used for grazing cattle. An estimated 14% of the world’s population (over 850,000,000 people) suffer from undernourishment while we continue to waste valuable agricultural land and resources to produce animal products, therefore obtaining only a fraction of the potential caloric value. Continuing this foolish management of our natural resources is simply not sustainable.