This is an excellent post on the 10 most common mistakes made by Photographers.

http://peterphun.com/blog/2009/05/01/top-10-mistakes-of-beginning-photographers/

The only one I would add to the list is not learning off camera lighting. Even if it is just using reflectors and available light, off camera lighting is very important, especially if you plan on doing portraits. Enjoy the post, it really is top notch!