composition
Mirror images and lines that form a new pattern.
Composition is the most important for a picture. Yet, it is the hardest to learn or to formalise in rules. All rules that one can give can also be broken in order to get a good picture. In general, I try to frame something as a picture contained in it self. So no people at, say, the right side of the picture looking to the right, i.e. outside the frame — what are they looking at then? Also, realise what it is that caught your eye: that you have to represent as well as possible.
With my bias to De Stijl, I like to follow horizontal and perpendicular lines in the composition. Someone once showed me a picture of a chanal in Brugge. The reflection was nicely captured, but I found the picture to be crooked. Not just the slight crookedness which sometimes occurs when not holding the camera still when pressing the shutter release (I tend to have an unbalance of about .5 to 1° clockwise), but really crooked. Well, he liked to take pictures at an angle. But hen, there was not a single line straight relative to the frame. And it took away from the serentity of the reflection of the gables in the water.