When there are moving elements within the scene, such as clouds or waves, a neutral density filter may be appropriate. This will serve to enable a long exposure to effectively blur out features in large parts of the scene, making the static elements more salient.
Fog and mist can also simplify a scene, focusing attention on the parts that remain visible. Snow and frost often do the same.
A polarising filter can, under certain conditions, help to define the outlines of clouds in particular. This can help draw attention to them as strong elements in a composition. The effect is most profound when shooting at approximately 90° to the light source.
Since our eyes tend to follow lines, either actual or implied, we can often use these to lead our eyes to a point of particular interest. Related to this is the tendency we have to follow the gaze of an animal or person to see what they are looking at. In this case you might position yourself and time your shot so that the gaze or orientation of a living creature leads your eye to another critical feature in a scene.
In addition to adding atmosphere, the morning mist helps define the church and patterns formed by the hedgerows and trees in this view of Widecombe-in-the-Moor, Devon. Canon EOS 7D with 24-105mm lens at 73mm, ISO 100, 1/30sec at f/11, tripod
A simple Dartmoor scene. The snow simplifies the foreground in particular, and following the gaze of the dog encourages you to explore other parts of the scene, including the distant Hound Tor. Canon EOS 5D MkIII with 24-105mm lens at 24mm, ISO 100, 1/20sec at f/16, tripod
A 3-stop neutral density filter has served to even out the water in this dusk scene at Hartland Quay in Devon. This effectively defines the two diagonal leading lines formed by the fore and mid-ground rocks. Canon EOS 5D MkIII with 24-105mm lens at 65mm, ISO 125, 30sec at f/18, tripod