It would be awesome if the user would be able to create folders for visual organization in tree views, tree list views and list views. These folders should not be actual items in the tree, but rather be a visual simplification for complex lists and trees.
Folder specific features:
View specific features
With this feature the user would find it easier to organize sometimes complex trees and lists into manageable folder structures, as the layout would be created by the user himself. The folders also make collective operations on items in them easier to handle, as the user can move or delete the folder with its content and does not need to select long lists of items for every operation. This would increase the usability of the trees and lists.