One thing I can't understand in WordPress is why doesn't it shows the categories inside their parents all the time? In Quick Edit mode, in the box Categories on New Post after you choose one and now in category list. In all these places is more difficult to administrate when the categories are showed in a common list.
I didn't like the new categories list in Categories editor.
And I thing that in Post Editor, when you are editing a post, on the box Categories, if you just duplicate the categories you choose you can show them on top and, at the same time, on the list without breaking the list as we see now.