It contains a good amount of all of the essential amino acids (ie the amino acids that your body cannot produce adequately itself). It also has calcium and iron. It's a pretty awesome nutritional food.
EDIT: to emphasize, the key advantage of quinoa is that it has a lot of all of the essential amino acids, which is very very rare among plants. I believe the only other plant with all essential amino acids is soy.
It's not very rare at all. It's rare if you compare it to grains, and while quinoa is not a grain, it is usually used instead of a grain. Many beans (e.g. white and black) and vegetables have a complete amino acid profile.
Even potatoes have a complete animo acid profile. Not in the most optimal ratio, but 10 potatoes/day gets you enough of all the essential amino acids. In fact you can live exclusively on potatoes if you add in a source of Vitamin A (like carrots).
EDIT: to emphasize, the key advantage of quinoa is that it has a lot of all of the essential amino acids, which is very very rare among plants. I believe the only other plant with all essential amino acids is soy.