It would be really handy if it were possible to drag n drop an album from the main panel into a category from the left sidebar. I wish to use it mainly to change quickly organize albums into genres (setting the file tags at the same time), but it just occurred to me while posting that the sidebar can group files in other different ways as well (not limited to genre only), hence the topic subject. I will leave the abstraction and specifics open for discussion. For me, just moving into genres would be enough but I think it would be possible to make this work for different kinds of tags as well!
edit: This is my setup below, for reference. I would use this feature by dragging the album covers from the main panel, with the ability to highlight/move several at once too.