It will be very useful if users could create categories from frontend.
There should be a button, just like Upload and possibility to set in ACL rights to create categories.
New categories should inherit ACL configuration from parent category.
The only problem I see is files path. This should be build from parent category path and an alias of new category.
IE.
Category: 2014
category path: /documents/2014
New subcategory name: January
subcategory path: /documents/2014/january
Next new subcategory name: John James
subcategory path: /documents/2014/john-james