Great article on icon usability by Aurora Bedford:
Other icons frequently misunderstood by users include the heart and the star. These icons are often used to represent favorites, bookmarks, featured items, and ratings from other users. Not only does the precise functionality associated to these icons vary from site to site, but these two icons compete with each other. As a result, these icons and their exact meanings are hard to memorize and interpret precisely. For example, the heart icon on the vacations listing site Combadi allows users to mark that they “love” a trip, but does not save that trip to a short list to ease future referencing. (…) In contrast, the interior-design shopping site Fab uses the heart icon as a way for a user to save that item in order to find it easily later.
Related: Realism in UI Design.
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