In the context of scalability a good analogy to software system is an enterprise. Assume that you need to design workrooms and production processes that will allow lots of personnel to work efficiently (read concurrently). Will you require all tasks to pass via a single man (logging)? How many toilets you need, and how will you place them (memory management)? Do you want to maintain certain ratio between programmers, testers and managers, or you will leave it to chance (various stages of processing)? The principles are all the same – excessive communications, dependencies, waits and centralization kill concurrency.