What functionality is missing? What is unsatisfying with the current situation?

I often struggle with finding the proper category to put an article into and I find it difficult to create categories that make consistent sense. For example, let's say I've got a category for books in my world. And let's say also that I have a category for magic items.   Now let's say I write an article about a magical book. Which category should that go in? I don't know and I don't like having to choose.  

How does this feature request address the current situation?

If I can assign the article to multiple categories, I no longer have to care about stuff like this.   While I am aware that I can tag articles in addition to using categories, I am not able to navigate my world using the tag-based features available to me. For instance, I can generate entire lists of articles with a given tag, but this is going to be done at page load. Meaning if I were to try to replace my Table of Contents in this manner, I would have to generate a link to every single article in my world at load. And of course anything with multiple tags has its link generated muliple times over.   Regardless, the primary concern for me here is actually finding my articles in the proper categories on the Articles & Categories page. No amount of tagging will help me there. Unless you want to add an "Articles & Tags" page instead. :)  

What are other uses for this feature request?

I can't think of any. There's probably some.