The Cat2Tag Plugin is something that was still missing for my photo blog: a way to work with WordPress categories like Flickr tags.
The most common tags (20 pieces, can be changed to all) are offered in a small JavaScript bar and additionally there is an input field for tags in which you can simply manually enter further ones (or click on the most common ones from the list). The default category is simply listed as a word in the tag list. New tags can be created simply by using the same. Very practical.
The hierarchy of the categories is of course not displayed - but this is not really interesting with tags anyway, the hierarchy does not play a real role with tags. Similarly, the tag description (category description) is not maintained, which must then be manually edited afterwards if you want to have something meaningful there (e.g. for the feeds).
The plugin still has one problem: it does not correctly convert the sharp s "ß" when shortening umlauts from tag names. But this seems to be a problem that WordPress also has - even when manually creating categories, the "ß" is not correctly resolved. One should therefore be careful with them. And of course, umlauts are not converted to their long form but to their base form - "ä" becomes "a". This makes the tag URLs somewhat problematic, as users need to know how the tag name is converted to the URL if they want to hack the URL themselves. But this is also a general problem of internationalization.
An idea for improvement would be an additional query string rule with which URLs with tag combinations (nature+animals) could be realized.
Otherwise, however, a really nice plugin with a very practical functionality for me.