I would say "following the good-photo guidelines" would be the biggy, whilst keeping post-processing to a minimum.
Rule of thirds for framing, good white balance, lighting and exposure, sharpness and contrast, etc would need to be of a high standard. Post processing would really only be used to correct minor issues such as touching up levels and colour balance as far as I would say.
I suppose composition would be a somewhat subjective element, except for objective shots like textures. You would need to find subjects and shots that you find appealing as well as branching out and finding shots that you think may be useful to other people and prospective customers.
For example, a podiatry company would be interested in shots of people walking happily on a beach or walkway with joggers. A sportswear company would be interested in people playing sports (obviously), partaking in exercise activities, leasurely outdoor activities etc.