Hmm... adding this to this thread because it kind of fits in with the idea of HotCat.js.
What about the ability to mass-add images to a category? I'm thinking of cases where people have created a gallery with images but none are categorised; this happens a lot and it's a pain to open every image and edit to add the category. Something like Magnus's category move javascript, where you can select images in a category and move them to another, but activated for galleries.
I'm thinking...