Letting the camera decide aperture/shutter speed. Even if exposure is correct, the subject might get blurred due to the camera's selection of shutter speed.
Putting the subject in the center of the photograph all the time. If you are doing videographic work, or documentary, instructional, etc. photography, then it might be preferred to have the subject in the center, but when you're going for artsy photographs that are supposed to evoke emotion, you often want the subject off-center. Similarly, the subject should rarely be looking straight at the camera in an artistic photograph. This is especially true if you are trying to create a "Thousand Yard Stare" impression. (The guy on the bottom left of the first picture is a good example. If he were the only person in the photograph, it would be a pretty awesome picture. If, however, he were centered, the picture would be mediocre at best.)
Not knowing how to achieve visual effects, such as depth of field, leading lines, etc., or simply not even attempting to achieve those effects due to a lack of knowledge on how to make a photograph look good.
Exposure and lighting, especially with a back-lit subject. Not waiting for or creating good lighting is a somewhat related issue. Sometimes pictures look much better if you take them in high-contrast light, or colored light (such as a sunset.)