I agree that the above suggestion would be great. But I'm just crazy enough to ask for even more!
Since categories and tags apparently behave the same when they are clicked on, it would be nice if WP allowed me to work with them in the same way. The same widget functionality and the same selection capacity in the post editors (quick and full). I much prefer the widget functionality provided for categories over what's provided for tags. But I prefer the tag selection functionality in the editors over that for categories.
If you look at my blog, you'll see that I use tags to group my posts like volumes in an encyclopedia, while I use categories to allow the reader to create unique groupings according to content. This used to be the other way around, but I was forced to switch them because of the limitations of the tag cloud widget. The unfortunate side-effect of this is that I now have to contend with that awful scroll box when I'm assigning categories to a post.