I always think that horizontal lines cutting straight across the picture are bad. The cut the energy out of the composition and stop the viewer's eye from looking all around the picture. In fact they can lead the eye right off the side. Lines running vertically or up the diagonals are much better.
This is perhaps even more important in a vertical shot.
This first shot is typical of many I try and take. It's nice but not satisfactory. The horizon is simply too straight, creating a horizontal line across the top third of the shot and making the picture look dead.
Compare with this.
The shape of the skyline is now much more interesting and lets the viewer look up and into the evening sky. Much to be preferred. The summit of Bowfell in this shot is what I call the composition's "resolution". It is the purpose of the composition. The viewer follows the path through the composition from foreground to background and when it gets there it expects to find some reason for the shot. The resolution is that reason, the reason why you have asked the viewer to look at that composition.
This isn't the same as the picture's subject. If you take a portrait of a person, the person is obviously the subject. Landscapes are often different, not having a single subject but being made up from the relationship between the different elements.