The natural tendency of most photographers is to shoot images in a horizontal (landscape) format rather than a vertical (portrait) format. There are probably at least two reasons for this. First, the most natural way of holding almost all cameras results in a horizontal format image. Second, we tend to see in a horizontal format mode (our field of view is wider than it is tall). Traditional art theory says that horizontal lines tend to suggest rest, calm and strength, while vertical lines tend to give an impression of height, grandeur and drama. By not turning the camera on its side and shooting a few verticals, you may be missing some great shots.
Salt Girl - enaandtheswan
Of course you can always crop your images to a vertical or horizontal format, but for maximum quality it's good to shoot the images as close to the final composition as possible. Besides simple aesthetic considerations, remember too that if you have hopes of one day getting your shot on the cover of a magazine, they usually want verticals!
Fall Bokeh - Kristybee
My Favorite Place - barbraziemerphoto