Research shows that web site visitors give up quickly if they encounter unexpected difficulties or delays. This is bad news for anyone who wants to do business on the web. It means losing customers before they make a decision to buy. Assume the user easily gets lost. It takes considerable effort to mentally map a site while exploring it.
Users can quickly lose a sense of where they are and where they've been.
A system of consistent and clear navigational markers can help users quickly recover their bearings. Assume the user is a web novice. Their web skills will be few, and much that expert users take for granted is counter-intuitive to the newcomer. Assume the user has a slow connection. Be mindful of download time.