Being a software developer I'm used to hierarchical structures. Understanding where a file is in a hierarchical structure of directories or moving or copying whole directory trees from one place to another is something I do every day. However, I have come to realise that a large percentage of people do not have this hierarchical concept in their heads at all when they use their computers. My wife, for example, has absolutely no clue that her pictures are stored somewhere within the C:\users\chrissy\... directory because she accesses them from a shortcut in the start menu.
Remembering a hierarchy requires a certain mindset and an intimate familiarity with the terrain of the storage system in which the hierarchy is contained. I am quite happy with the internal structure of my disk drive and, because I use certain rules and regulations of my own, I am able to find a file within that structure very easily and quickly. I'm also sure however, that someone unfamiliar with my machine would find it very difficult indeed to find a specific file. Similarly, I find it difficult to find files on machines that have been set up by another people.
There seems to be then, a very nice analogy which can be drawn between users of Computer Systems and users of street maps in unfamiliar towns. Someone who has lived in the town all of their lives and knows all the back streets and alleyways can probably get from A to B quickly and efficiently whereas someone from out of town might struggle to navigate anywhere off of the high street.
Underground railway metro maps including those iconic maps of London, Paris, New York, and Barcelona to name but a few are shining examples of concise simplicity which are designed to make entire cities less daunting and more accessible to the millions who are unfamiliar with their hierarchical structures. Windows 8 Metro has exactly the same design goal as those maps.
In the same way that the metro line is linear and as well defined stops along the way, an application designed for windows metro must have the same well defined structure. One should always begin in a familiar place, being able to move along the line easily, and be able to move back to stations that you have already passed simply by going back in the other direction. There are, of course, a few examples where this principle should not be upheld religiously, in particular the example of credit card payments should never allow the user to navigate back through the pages where information is entered once a transaction has been registered. This is a rule which many web sites ignore to their detriment, possibly because the web style back button is inadequate for changing the rules of going back in such a sensible but non intuitive manner.
Metro is a "Design language" more than it is a set of APIs or software libraries. There is a great deal about Metro that is not expressed at all in the software but which is implied in the design guidelines. Metro can be considered as a start menu on steroids but with a fair dose of Valium thrown in to calm things down a bit. Think calm, think concise, think Metro.
