Vertical and Horizontal Pictures
Unless you are in a very specific genre that requires you to shoot only in either a vertical or horizontal orientation, have a good mix of both and demonstrate you ability to work both ways.
Natural and Artificial Light
Understanding light and what to do with it is a keystone for any good photographer. An editor is going to want to see your ability in understanding how to use natural light to capture a scene more dramatically, as well as if you can navigate around a studio and work with artificial light. A third source here is also the use of fill-flash outdoors. While digital photography and enhanced metering systems in both cameras and flashes has made this technique easier to master, one or two superb examples of using flash outdoors are a good addition to your portfolio.
Creativity and Responsiveness
Photographers have to be both creative and responsive. Creative is being able to see good picture when it is front of them. And responsive is being to anticipate and take the picture at the right moment. The type of work for which you are interviewing will dictate how much of each you need to have in your portfolio. But, at a minimum, you need to be able to show you can be both.
Your Own Style
This is probably the most important component of any successful portfolio. With the increase in microstock agencies,there's a possibility editors would be able to fulfill their photo needs by looking through those archives. Creating your own style or developing you own "eye" will be the intangible factor that sets you apart. If an editor is interviewing you, he wants to see what you can do, not what you can replicate by looking at other peoples' work. Over time, as you continue to shoot more and more pictures, your personal style will start to emerge. Include a few black and white photos for variety, as well.