I have two suggestions for better menu management (not the backend administration menu).
Remove the current menu manager. We don't need it.
All we need to do is to set what page we want / page we do not want to show in the menu structure.
Then implement Flávio Ricardo's suggestion of showing / hiding child elements (http://wordpress.org/extend/ideas/topic/management-pages)
The current menu system is ok, but thre is room for improvement
- When a page is deleted, the corresponding menu item (if exists) should also be deleted
- One click button to generate first main menu as a copy of the page structure would be nice.