This is a great question and I get asked it often. Now the specifics may vary slightly with your discipline, but I feel your design portfolio should demonstrate five specific characteristics:

1. The first is that you are a creative thinker.
2. A strong knowledge of the principles and elements of design.
3. A knack for organizing various elements in a specific space, whether it be on a piece of paper, a computer screen or in a room.
4. A good knowledge and use of color , and
5. An eye for composition and grid structure.

Click here for an example of a great graphic designers portfolio. For the young designers make sure you go over these with a Mentor or trusted friend; for you seasoned professionals out there, drop me a line to let me know your thoughts. I look forward to your feedback.