Answer:
Protein! Most meat eaters actually get more protein than is necessary, and a whole lot more cholesterol. Vegetarians get plenty of protein in soy products, nuts, and beans, while getting other necessary nutrients that are absent in meat (not to mention the hormones etc that is used in processed meats).
The one thing that vegetarians tend to be deficient in is vitamin B12, and the B complex, which allows the body to metabolize food easier and break carbs into energy. The best suggestion for this is sublingual (NOT swallow-able!) vitamins, since they are more likely to absorb that way.
Vegetarians also tend to eat too many carbs and cheese in trying to make up for the "substance" that comes from having chicken or steak on the plate. Therefor, they generally gain weight the first time around. However, if you watch enough to get the full serving of fruits/veggies/grains...you will find you are much more balanced because you put more veggies on your plate than meat!
Good luck!