Tags and categories are two different things. Categories are hierarchical. Tags are not.
But why can't they be?
(Hierarchical, by the way, means that they have parent categories and child categories. Music is the parent category with subcategories for Concerts and CD Reviews.)
I would absolutely love to have hierarchical tags. Case in point, my tag cloud. It's a mess! If I could organize these tags into movies, classes, actors, bands, it would be so much more useful. As it is now, you can't find anything.
"So? Just make subcategories for each of them!" you say. "Yeah, riiight!" I say. How am I going to categorize my posts with a list of over a thousand different categories and sub-categories and sub-sub-categories? And wouldn't that take a little while load?
The easiest way I can see to do this is to tag things as "movie:a-hard-days-night" rather than just "a-hard-days-night". But that's going to look ugly on my tag page. Plus, the address will be "/moviea-hard-days-night/" rather than "movie/a-hard-days-night", as it should be.
I'm no plugin author, so I'm really hoping somebody has done/will do this. But if need be, I'll figure it out myself. Because these "flat" tags just plain blow.