The Samba Team and other Linux development groups are working hard to clone Active Directory compatibility for Linux and Unix. This article will give you a taste of the complexity. Their design philosophy is based on the single user computer. Their programmers are hired right out of college and have no business network experience. Microsoft's management is very aware that security does not sell their product - features sell their product.
Security requires check points and firewalls which interfere with tight integration.
Security is always annoying to users. Secure networking needs to be built in from the start, it can't just be added on later. Designing effective network security requires experience and a deep understanding of network usage. The more features a piece of software has, the more likely it has security holes. It takes little skill to compromise a Microsoft system.