Single authority principle
One of the biggest issues in today's IT architectures is overengineering. Excessively complicated solutions are bound to be less reliable and secure. Any other component in a solution is potential point of failure. Well, not necessarily in terms of reliability (clusters, you know) - but certainly from security point, as it adds potential vulnerability and attack point.
And it creates some interesting issues. An example is a popular approach to identity management: using HR database as a "source of truth" about company staff. The corporate directory (your AD, or NDS) is populated from the HR database, using middleware. If you're using same corporate directory for controlling access to the HR database, that will result in an access management chicken-and-egg problem: who is the authority? Besides, now attackers have two targets for taking control over entire enterprise infrastructure (and middleware, the identity management system, is the third equally important). The approach avoiding that situation is developed centuries ago by the world's military: use single authority.
Using HR database as the source of truth does make sense, as it contains information about those who are paid by the company. However, it so happens, incidents that are most difficult to investigate are not caused by paid staff using their own accounts. Few years ago the security industry was shifting its focus towards malicious insider. But recent events prove that classic intrusions without apparent access abuse are still big threat. Take TJX and their insecure wireless network. Nevertheless, we should soon see more close integration between business systems like HR, identity management solutions and corporate directory. Oracle is making steps in that direction already. I like Microsoft's Active Directory and would like to see some effort in that echosystem as well.
But that is a move towards identity and access management based on military principle. It would be very interesting to see a system that is based on democratic principles. That is unseen so far but may well be an interesting change in the enterprise space.