In Moodle 1.9 Beta 4 + (2007101508.08) when I assign the course creator role in category context it is impossible to create a subcategory.
This worked before in 1.8.4.
The capability of creating categories is granted to the role of course creator on site context. (default settings)
After assigning the role it is possible to join the editing mode in a category, and the button for creating a subcategory is shown.
Clicking this button will display a form, requesting the information for the new subcategory.
But now, a click on the submit/save button won't show the expected page, saying "change saved", but the editing form again.
It's unclear if the feature, giving a course creator the capability to create subcategories on category context and below, was taken away since 1.8.4 or this is a bug. Anyway it is a feature which should be available. Otherwise you only can create a course creator as a global role, making not much sense due to the fact that such an assignment works for the whole site. Many want restrict the permission of creating categories on a defined region like a category context and beneath.