As a directory server, OS X can manage a mixed Mac, Windows and UNIX environment. As an application server, it can do pretty much anything, the only problem being migration. And if you have that one Windows server app for which there is no Mac equivalent and it was written specifically for Windows (not a cross-platform Java/Ruby, etc., app), you can either run it on a Windows box in the network or virtualize it within one of the OS X servers.
Then you can watch your TCO go down. For example, Apple has the ability to remote to systems, do remote unattended application installs and updates, share screens with the ability to hide actions from the user, do remote encrypted large file copying, list systems according to almost any hardware or software criteria, get application usage and user reports, and monitor with hardware, software and inventory reports. You can also script actions, create workflows and use Spotlight to search the remote systems.
You can do all that in the Microsoft world (except the Spotlight search) with a bunch of different utilities. Some of those are free or included, and some cost thousands plus client licenses. With Apple it's called Remote Desktop, $1,000 for unlimited clients.
I understand. Now, if I have applications that have to run on Windows systems, and I go the route of doing it in virtualized Widows clients running under the Mac, will OS X manage those Windows machines? If not, what just happened to my TCO?