Web usability stays interesting because, as technologies evolve and users gain experience, standards and best practices evolve over time. In the past I've recommended against relying on breadcrumb navigation for web applications. Breadcrumbs show a "drill-down" hierarchy, but most users do not construct the mental model shown by the breadcrumbs. Yes, users sometimes click breadcrumb links, but they tend to treat them like any other links on the page, since they don't learn the structure presented.
Today, however, Jakob Nielsen writes that more people have been using breadcrumbs in his usability studies, and furthermore, the use of breadcrumbs in Windows Vista means their importance will grow. I feel better about breadcrumbs ... they're very useful, and as people learn what they really mean, website usability will benefit.