I think what makes a good portrait picture is that it should be clear and show the persons features well.  I think it is sometimes hard to get a good shot, because it depends on where the picture is taken, what amount of light you have, etc.  I think that good lighting and a decent camera are needed for a good picture.  Also, they should be a staged picture not a candid shot.  A portrait should also have a pleasant background.

This picture is great because it has a nice background, good light, and the person is centered.

The baby is centered and there is a nice background/props.