Good photography, regardless of color or black and white, needs to create an emotional connection with its viewers. This is what is referred to as impact. Not all images will create impact in all people, but if you know who your market base is, you need to figure out how to connect with those people and clients.
Within color photography, the image has the power of the color to create that impact. A beautiful warm sunset over a sand covered beach will create more emotions then that same location taken when the sky is plain and the light is dull. B&W does not have the impact of beautiful sunsets to fall back on, so it needs to create that impact in a different way.
I personally believe that my best black and white photos are taken when I deliberately set out with b&w images in mind. I have a different mindset about what I shoot and how I shoot it. I look at the details and textures rather then the colors.
I also use light to enhance the impact. Light will create the shadows that will define shapes and details. The best shadows are not taken at sunrise or sunset, but often at different times throughout the day depending on how I will be using those shadows to work with lines and shapes within my composition.
Within landscape photography, I am always looking for interesting cloud formations or stormy skies to create added drama to my scene. Big billowing clouds have shadow and depth, with tonal ranges that add depth to the overall composition. Feathered clouds give a scene a sense of motion and life. Subject matter in many cases can almost become second to the clouds, but clouds without subject matter can be boring just as subject matter without clouds can be plain.
A plain blue sky just reproduces itself in B&W photographs as light gray. This plain gray can work fine if the subject itself has impact…such as a strong lighthouse or a tranquil tree in the fog. But in most cases in b&w, if the subject matter is not strong, the sky should be.
For people photography, impact can be achieved by the emotions in the face whether it is one of tragedy or joy. It can also be achieved by the story the scene tells. What is happening within the frame of the camera. Why is the person happy? What is making the person thoughtful? These are the questions that can help you decided if your image has impact.