Windows Server 2008 Active Directory Domain Services includes a new feature called Read-only Domain Controller (RDC). It enables better administrative management and enhanced security.
For example, on better administrative and security level, it requires no password in the default platform which can lower the risk of passwords being tracked by others in RDC forum.
For a more detailed explanation, you can watch the video (Active Directory Domain Services (AD DS) in Windows Server 2008 Technical Overview)