One quick expansion of the functionality point:A really critical difference that stands out for me in many of these discussions of tagging (not only LibraryThing vs. Amazon, but also librarian reactions to tagging in general and things like PennTags in particular) is the degree to which people will compare environments which are largely organized by and searched by user-added tags vs. those in which tagging is supplemental to an existing categorization or navigation scheme. Amazon already has categories (I won’t risk the justifiable ire of catalogers by saying it already has subject headings); tags enhance rather than order its contents.