Sometimes, curves might be used to make colours stand out.
Diagonals
Diagonal lines consist in the photo, sometimes crossing over eachother.
Phi Grid/Golden Ratio
The Phi Grid, or the Golden Ratio, gives a template to the photo and frames objects.
Figure in Ground
The subject of your photo may be darker or lighter than the background, this creates the figure in the ground effect. When you take the subject out of the picture, the picture seems incomplete.
Triangle
Some areas of the photo may be aligned as a triangle, or some spaces resemble a triangle.