Bookmarking and tagging

The more I worked on the bookmarking and tagging project for LIS 2600, the more I realize the different functions I want tags to have. I need to tag not only for subject, but also for format. I want to know if it's a blog, a pathfinder, an article, a wiki, or a general website. This tells me that format has an impact on usefulness and content of an item. I think I should have known that, but it's hit me in a different way.