There's a vast logical difference between the domain system and the directory. Administrators must understand that before a conversion begins. This restricts the number of administrators, since it isn't simple to give an administrator rights to administer just part of a domain. Organizational units also reflect practical considerations-it's extra work to administer a domain with users in two locations connected by a slow WAN link. With a directory structure, everything changes.
Organizational units are solely logical.
A directory tree can be divided by geographical regions, with offices in Chicago, Los Angeles, and New York having separate subtrees. Because the directory is actually a database of network resources, everything on the network is a database object. Users, groups, printers, and PCs are all objects. With Active Directory, you can assign administrators' rights at granular levels. Such a structure also provides a way to create administrative roles that grant the right to manage network objects based on a job title rather than having to grant rights to an individual.