1. Sharpness. In amazing landscape photographs, everything that's supposed to be sharp, is. Conversely, everything that isn't supposed to be tack sharp… well, it isn't. That means a photographer who knows how to choose a sharp aperture—both for lens sharpness and to create the most appropriate depth of field. The sharpest aperture is usually two to three stops from wide open, and the only way to know for sure it to test your lenses. Sharpness also means photographers rely on a tripod when they make landscapes, even if they're not using a particularly slow shutter speed. It also means that when motion blur in trees and/or water is the desired effect—as it often is in landscape photographs—not only is a tripod a necessity, so is an understanding of how shutter speed affects sharpness and blur. Lastly, sharpness means more than anything a correct focus. One of the best ways to focus correctly in a landscape image is to avoid the use of autofocus—which may incorrectly choose its focus point in the vast and static landscape—in favor of a precisely controlled focus selected—and verified—by the photographer.
2. Clarity. Usually, when it comes to great landscapes, clarity doesn't just mean a technically clear image—although that's certainly important. Just as important, though, is a compositionally clear image, and an image with a clear motivation. That means the photographer had a reason for choosing this particular composition, this particular camera angle, for including these elements while eliminating others. The message is clear, so to speak, because the photograph—be it big and grand or small and detailed—is made with a purpose. In these cases the photographer has created a composition that allows the viewer's eye to move seamlessly through the scene, focusing on the subject as the explicitly intended. It's a clarity of thinking that creates this visual clarity, and it primarily comes from planning and forethought. Work carefully, and your pictures will be clear.
3. Impact. Impact can come in many different forms. It might be, "Wow, look at that bright color!" or perhaps it's, "Look how calm and quiet that quaint little scene is." Impact doesn't have to be a big boom, but it is always key to great landscapes. What defines the impact is really up to the artist. It's a measure of success, in a way, of the image. For instance, Ansel Adams often used drama for impact. It truly was a wow factor. Many other photographers use subtlety (of tones, light and shadow) to achieve a similar level of impact—even though they're going for an entirely different effect. No matter how you strive for impact, the key is to pick your ideal destination and then take the photo there. If it's a calm scene, make it especially so, and use compositional and processing techniques to help you achieve it. For big and bold, go over the top and make it shout, using different techniques to achieve the same type of impactful end. Ultimately you just have to avoid the muddled middle ground (of technique as well as motivation) in order to create landscape photographs with a strong and clear visual impact.