As we mentioned in our review of Mac OS X Lion, Apple either chose to or was forced to make changes to how the SMB file sharing protocol was implemented in OS X. This had few implications for file sharing between OS X and Windows, but one casualty of the switch has been the ability for OS X Lion Server to run as a primary domain controller (or a backup domain controller) for Windows clients: basically, if you're hosting a directory with users and groups using Mac OS X Server, Windows clients won't be able to connect to your server and take advantage of these services.

You can read more about directory services, Lion Server, and its ability to serve Windows clients in our in-depth piece on Lion Server.