I'm in need of this functionality too, which for a multi-user system should be a reasonable thing to aim for 'out of the box'!
I only wish I had the abilities to help make it happen....
What I'd like is the following:
1) Ability to restrict users to a given category (categories) and their sub categories (optional)
2) The ability to restrict what users can do with pages
3) To be able to define classes of uses with certain restrictions (e.g. define a class of users as 'The Simpsons' and have them unable to post in 'The Flanders' category).
4) The ability to restict users to editing a particular page (and to create new pages with that page as a parent, or more distant ancestor).
5) All the while maintaining authoring/publishing roles, with a 'submit for publishing' button for editors, which emails the relevant person/people with 'publish' permissions!