Complex URLs

"Even though machine-level addressing like the URL should never have been exposed in the user interface, it is there and we have found that users actually try to decode the URLs of pages to infer the structure of web sites. Users do this because of the horrifying lack of support for navigation and sense of location in current web browsers. Thus, a URL should contain human-readable directory and file names that reflect the nature of the information space."

Source:
http://www.useit.com/alertbox/9605a.html

Further information:
http://www.w3.org/QA/Tips/uri-choose