The Longhorn Identity System (codename "Castle") is a long-awaited feature.
It replaces the ancient Workgroup model in home Windows installation with something that's more lightweight than a full-blown Active Directory. Been hoping for something like that. (And by "hoping", I mean "idly wondering about it at some point in the past").
Too bad it's a Longhorn feature. It's also apparently based on WinFS, which has been pushed back to, what is it now, 2050?
Here are some slides about it from PDC '03. Does anyone know if there's any newer information about this?