As anyone who has followed my past work knows, software architecture is a particular interest of mine. I find the subject fascinating, but my interest is not entirely selfish.
Understanding architecture, and the trade-offs that different architectures imply, is an important part of any software project. Whether you're discussing a Content Management Platform like Drupal, a language like PHP, or a particular web site, having a solid understanding of the "big picture" is crucial not only for building the system right in the first place but for communicating that architecture to others.
To be able to speak and think about the design of your system properly, though, you need to understand the trade-offs that come with it. There is no such thing as a free lunch, and designing a system to be powerful in one way invariably tends to harm it in another. It is important to know what your priorities are before you start building; and in a distributed collaborative environment like Drupal to all agree what those priorities are, at least to a large extent.
Let us therefore examine those priorities and the trade-offs they require.