When someone calls a person, a story, or an idea one-dimensional, they are not pleased. They mean it is too simple: it shows only one side, and it misses the rest. A one-dimensional character in a film is just “the angry man” or “the kind mother,” with nothing else going on. A one-dimensional argument looks at money and forgets feelings, or the other way round. The phrase is a criticism. Though actually, the picture comes from maths: a line has one dimension, a flat shape has two, a box has three. People borrowed that picture to say “flat, thin, not fully real.” In a science class the maths meaning is still alive, but in newspapers and everyday talk the cross meaning is the common one, and it is almost always negative.