[0]http://www.ncbi.nlm.nih.gov/pubmed/19562864

Here's just a few lines from the full text of your link

Vegetarians, and particularly vegans, tend to have lower blood levels of EPA and DHA than nonvegetarians (15).

Zinc intakes of vegetarians vary with some research showing zinc intakes near recommendations (32) and other research finding zinc intakes of vegetarians significantly below recommendations (29,33).

Some studies suggest that vegans who do not consume key sources of iodine, such as iodized salt or sea vegetables, may be at risk for iodine deficiency, because plant-based diets are typically low in iodine (12,35).

In the Oxford component of the European Prospective Investigation into Cancer and Nutrition (EPIC-Oxford) study, the risk of bone fracture was similar for lacto-ovo-vegetarians and meat eaters, whereas vegans had a 30% higher risk of fracture possibly due to their considerably lower mean calcium intake (38).

Low vitamin D intakes (42), low serum 25-hydroxyvitamin D levels (12), and reduced bone mass (43) have been reported in some vegan and macrobiotic groups who did not use vitamin D supplements or fortified foods.

The vitamin B-12 status of some vegetarians is less than adequate due to not regularly consuming reliable sources of vitamin B-12 (12,46,47).

No unfortified plant food contains any significant amount of active vitamin B-12.

Vegetarian diets are typically rich in folacin, which may mask the hematological symptoms of vitamin B-12 deficiency, so that vitamin B-12 deficiency may go undetected until after neurological signs and symptoms may be manifest (47).

Vegetarians did not meet dietary standard (in at least one country) for:
● vitamin B-12 (in the United Kingdom);
● iron (in the United States, for both vegetarians and omnivores);
● folate (in Germany, though lower rate of deficiency than among omnivores);
and ● zinc (in the United Kingdom).

Infants of vegetarian mothers appear to have lower cord and plasma DHA than do infants of nonvegetarians (70).

Breast milk DHA is lower in vegans and lacto-ovovegetarians than in nonvegetarians (71).

Vegetarian athletes may have lower muscle creatine concentration due to low dietary creatine levels (92,93).

Some, but not all research suggests that amenorrhea may be more common among vegetarian than nonvegetarian athletes (94,95).

Not all aspects of vegetarian diets are associated with reduced risk for heart disease. The higher serum homocysteine levels that have been reported in some vegetarians, apparently due to inadequate vitamin B-12 intake, may increase risk of CVD (111,112) although not all studies support this (113).

Although very little data exist on the bone health of vegans, some studies suggest that bone density is lower among vegans compared with nonvegetarians (164,165).

Vegetarians can, however, have risk factors for dementia. For example, poor vitamin B-12 status has been linked to an increased risk of dementia apparently due to the hyperhomocysteinemia that is seen with vitamin B-12 deficiency (188).

TL;DR:

The complexity is there. It is possible to eat a balanced meatless diet, but at the same time it is quite easy to screw it up, and the consequences may show up after a few years and take time to undo.