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The term "subject" causes a lot of confusion, probably because people often assume that a subject is what the photo is 'about', which is not always the case. A subject is also not necessarily a distinct, separate element. I prefer to think of it instead as an 'apex' or 'home base' for the eye, a point of visual resolution. It's the area of an image where the eye wants to land between explorations of the frame. Without one, a photo tends to look unresolved, and the viewer quickly loses interest. In the seascape that Rob posted, there is such an area to 'land', which Ben pointed out. Some landscapes have a more clearly delineated apex (for lack of a better word), such as a lone tree in a field, an isolated waterfall, or a sun twinkling at the edge of an arch, but the main point of interest could also be a part of a larger element: the snowcapped peak of a mountain that is the tallest in a long range of peaks, an area of an expansive plain picked out by a shaft of sunlight, etc.
The basic goal is to give the photo a sense of hierarchy. Many compelling landscape scenes have multiple points of interest, but they usually work best if they don't all compete with each other; if one of them clearly trumps the others, then the photo is likely to hold together pretty well.
As for rules: Every rule/guideline is based on avoiding particular problems, so you need to understand the reasoning behind one before 'breaking' it--if the photo will not suffer from that problem, then you can get away with breaking the rule. For example, if the strength of a photo lies in a remarkable all-over pattern (a sort of 'horror vacui' effect), then it may work without any 'apex' to pin it down at any particular point. Another example would be the placement of a strong line across the center of an image: this 'rule' exists because such lines tend to bifurcate the scene, making it look like two separate images juxtaposed, and the eye will tend to ping-pong uncomfortably between them. Conversely, if there is something to tie the two realms together--common colors, a reflection, a strong vertical element that extends over the line and 'stitches' the two halves together, etc.--then the placement of the line will matter a lot less.
I have to admit that I really enjoy this particular topic and have just plagiarized myself by cutting and pasting bits from replies I've written over the years to the same question on other forums.