Rhythm is one of the keys to great landscape design, and there are several types of rhythm that make design more powerful. Whether is be alternation, crescando, descending, or one of many others this is an excellent tool for designers.
The other important (or one of the other), thing is understanding the space between the actual elements that are used. The voids are just as important as the mass. It is this interplay between mass-void that create the defined space. The tricky element to understand is that each type of mass/structure/plant/stone . . . whatever; needs its own type of void to be successful. The requirements also differ depending on the style, site and needs of the participants who will use the space.
Tricky-yes. Rhythm is something that should be continually thought about, and used accordingly.