Due to the way different lenses or zoom settings affect perspective, it pays to know what focal length to use for certain subjects.
In landscape photography for instance the idea mostly is to show a lot of the scene in front of you. This would best be captured with a wide-angle lens since this is able to show the wide open space before you. Because wide-angles open up perspective, some foreground interest will be necessary to avoid creating an "empty" photograph.
For the same reason a wide-angle should not be used for close-up portraits, as the facial features of a person will be distorted beyond recognition. If you definitely want to use a wide-angle to show a person in its surroundings, make sure he or she is not situated directly in the foreground or at the extreme borders of your image, as this is where perspective distortion will manifest itself most strongly. Keep the person away from the extremes of your image and a natural looking portrait will be the result.
When using long tele lenses be aware that with distant subjects, haze and mist can affect the apparent sharpness of an image. A skylight filter will lessen this effect, or shoot early in the morning before the sun has warmed the ground. Take great care when focusing with a long tele as depth of field will be very shallow. A long telephoto at maximum aperture and focused on a subject 5 meters away will only give a few inches depth of field.