People and Hierarchies
SAP Design Guild covers hierarchies in depth. Part one of their series has an especially good reminder on considerations to bear in mind when designing anything with a hierarchy -- like a typical site map.

"people have problems with abstract hierarchies. They cannot create a suitable mental model for them because the system seems artificial, and often they do not understand what the categories mean. Deeply nested hierarchies cause even more problems because people get disorientated. People already get confused in mazes, where they only have to remember a larger number of binary left or right decisions. It is even easier to get lost in complex application structures, hypertexts, Websites, or the Web as a whole, if there are no "anchor points" where they can regain their orientation. People need to know where they are, why they are there, where they came from, and where they can go."

[via InfoDesign]