(Active Directory trigger warning) Even when the number of users goes over 10k I'm not sure it's worth it. I've lost days chasing down random errors to do with DNS or a misbehaving DC... when the error reference looks like http://www.selfadsi.org/errorcodes.htm (although I wish I'd known about MS's err.exe at the time)

And that's just when you're using the recommended connectors (ODBC/ADO.NET). FSM help you if you're using JDBC and the vendor of the $20k+ product hasn't tested against Server 2012.

I wonder if there's any money in providing an LDAP/AD -> REST server.

http://stackoverflow.com/questions/2453769/active-directory-... might be an idea, too.