URL as UI
Users continue to type and guess URLs and domain names, so Web usability can be improved by better URLs. In the long term this machine-level addressing scheme must be hidden.
This article from 1999 about how to write good URLs is awesome. Some tips:
Write in lowercase
Check for typos
URLs should be permanent
Keep URLs short
But Nielsen's overall view is that URLs are an important part of user experience. They contribute to a website visitor's sense of trust and geography. One of Neilsen's rules dictates that a user should be able to "hack" of the last segment of a URL to get to the parent page.
Ironically, in 1999 Nielsen thought that URLs and search engines would soon fade into obscurity as better solutions to information management emerged. Lo, 25 years later the URL remains more important than ever. Sometimes the simple solutions is the best one.