Defining access to or rights of a user, isn’t the easiest thing. Beside some minor problems two main problems (in my opinion) still need to be solved:
1) A user needs an easy to use interface to define his rights
2) A user needs to be informed adequate about the impact of the rights he is about to change.
A really nice solution for the problem domain of 1) can be found by Quasthoff ([1],[2],[3]) sowie Müller Prove [4] und Passant [5]
[1] Enlightenment 2.0: Facilitating User Control in Distributed Collaborative Applications
[2] Who Reads and Writes the Social Web? A Security Architecture for Web 2.0 Applications
[3] Why HTTPS Is Not Enough – A Signature-Based Architecture for Trusted Content on the Social Web.
[4] Modell und Anwendungsperspektive des Social Tagging