One thing that many amateur photographers try to do is fill up the frame with many diverse and interesting elements. This is usually contrary to a pleasing composition. Sometimes (as with some landscape photography) the subject will fill the entire frame, but with most subjects you want to pick one or two things and isolate them from everything else. This creates a feeling of depth.
Take your 3rd photograph as an example. The subject is the steel supporting element. The subject is isolated from the rest of the picture by contrast because it's darker underneath the bridge, and the photograph is exposed for the subject. I'm not saying you should have, but there are other ways you could have isolated the subject even more. You could have underexposed it which would put more emphasis on the shape of the object. The reason the subject is interesting is not because of its color or texture, but because of its shape. Another way you could have isolated it is by causing the background to go out of focus. If you use a longer lens or longer zoom setting, placed farther back from the subject, and a large aperture setting (smaller f stop number), the background will start to go more out of focus.
Most amateurs starting out want to photograph exactly what they see. However, the problem is the camera is renders a two dimensional image and most subjects are 3 dimensional (or 4 if they are moving). So the challenge is not to photograph what you see with your three dimensional view, but rather to create dimension in the photograph by drawing the eye to the subject. You can do this by color, contrast, focus, framing, and/or by including an element such as a path which draws the eye where you want it to go. So rather than trying to capture what I see, I look at photography more like painting. I want to build my composition by placing items inside it depicted in such a way to provide depth (or sometimes lack of depth which is another subject), but more importantly I want to tell a story about whatever it is that finds itself in front of my lens and I want the viewer to be drawn to that subject.
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