I think non-Americans need to realize that the sandwich is one of the most important and well-developed parts of American culinary tradition. It isn't just a food we occasionally eat, it is the one type of food that every American of every walk of life is likely to be eating on a regular basis. It is one of the staples of the American diet, and has thus seen a tremendous amount of innovation and elaboration.
People in the thread have commented that "American bread must be really bad to force you to put so much on it", but that is actually a really mistaken line of thought. The sandwich is seen as a major dish in of itself, not a compliment to something else. It is the center-point of either lunch or dinner (and sometimes breakfast). As such, people bake bread for the express purpose of making sandwiches. Bread that people use for sandwiches and bread that people eat on its own are two different products. For example, I would serve French bread or dinner rolls with butter as a side dish for dinner, but I generally wouldn't use those to make sandwiches. Instead, I would use sliced bread or a specialized sandwich roll. For a sandwich, bread is just one of the ingredients, but you would only use the right bread for the right sandwich.
One of my favorites is the the clubhouse sandwich. It is a multilayered-layered sandwich: there is a third piece of sliced bread used in the middle of the sandwich to separate different layers of meat and vegetables. It is usually made with turkey, bacon, tomatoes, lettuce, and mayonnaise. It is usually served cut into quarters, with the slices held together with toothpicks to prevent them from collapsing. It is delicious. Takes some time to make though.
Though we do have some really good simple sandwiches. One of the best is the French dip sandwich, also called the beef dip sandwich. It is a simple sandwich consisting of thinly sliced roast beef on a sandwich roll or baguette, served au jus. When you eat the sandwich, you dip it in the cup of jus before taking a bite out of it. Simple but sublime.