A good photograph is made up of several elements. If one of those elements is lacking then the photo is not as good as it could be. Some important elements are technical (focus/exposure/DoF), composition, color scheme, and subjectively, subject matter. Another very important element to consider when creating a good photograph is lighting. It is very often overlooked and we snap the camera shutter more times than naught without giving the lighting much consideration.
If you want to drastically improve your photography then I highly recommend studying light and how it interacts with your subject. By learning this you can learn to manipulate that light to improve your images. I think a lot of photographers (especially the readers of this blog) know a little about light, that midday sun creates harsh shadows, the better light is morning or afternoon and will reduce the harshness, or that large shade areas will produce nice soft lighting. However there is much more to light than that especially when lighting still life or portraits. Armed with this knowledge you will know how to better light your subject in a given situation and get the best image possible under the circumstances.
A book I highly recommend on lighting is called Light - Science & Magic 3rd ed by Focal Press. It is written by Fil Hunter, Steven Biver and Paul Fuqua. The book will teach you how light interacts with matter and how you can modify the light to produce better images. The book is on it's third edition now, and not that light has changed since creation, the book has been updated to include new technology and just as important an updated modern look.
(more details in the full post)