When discussing usability, I tend to bounce from element to element - menu this, font-size that and so on. The last year or two introduced some non-design elements to the conversation, like AJAX, accessibility and semantics. Not until the last few months, however, have I considered URL construction an important part of usability. SEO, sure, but usability? Never. As URL-rewriting becomes the norm, more and more people tend to notice the semantic structure of well-formed URLs. For example, it doesn't take a developer to notice how the following URL relates the content on that page:
My guess: Muzak's portfolio entry. If one were to see the link URL above, they could easily come to that same conclusion.
It's important to know that some people may be placing much more emphasis on your URLs than you intend for them to. Some people may even try to navigate your site by manipulating the URL itself, rather than find the link they want to follow. Also, URLs may soon become a common form of user-interface (see URL as UI). Others may disregard links they see in emails or other material due to their ambiguity or malformation (spaces and special characters, for example). The widespread use of email links for phishing and other crimes also cause users to scrutinize links more heavily. Taking the time to construct usable, well written URLs can help avoid these problems, and, in turn, avoid losing potential customers or users.
With that said, here are a few tips on good URL writing that are pretty standard:
Use whole words instead of abbreviations.
Avoid spaces and non alpha-numeric characters (including punctuation)
Use index files in all directories
Use all lowercase letters
While some may have never crossed your mind:
Avoid using underscores †
Use URL-rewriting to capture common typos and redirect them to the correct page.
Use URL-rewriting to remove query-strings.
These tips not only help with usability, but are also beneficial in increasing your site's search-engine performance and product portability.
† To visually separate words, use hyphens instead. When a link exists with underscores and is underlined, then it appears to have spaces in it to the end-user. This also helps score better with search engines.