Directory Services are where the big battles are raging.
We're rolling out a directory-enabled network on our campus (we're a small technical college) and the initial decision to have AD/W2K do all the back-end heavy-lifting is looking worse and worse.
We have a heterogenous network (Various Windii, Linux, Solaris, Mac OS, OS X) and need to support everybody's desktop with file/print/authentication/DNS/DHCP/routing/etc. The more we look at AD, the more potential issues we're going to run into if we roll it out.
My current suggestion is to move Linux into most server roles (like classroom file/print/router) and use NDS/LDAP as our main directory service platforms. I'm shopping for a *NIX-based copy of NDS (renamed eDirectory) even as we speak.....