There were few operating systems that handled loose-clustered networking as elegantly as VMS. Want to centralize user credentials? Easy, just place SYSUAF.DAT on a shared volume. And since the files could have structure, you could lock individual user records for editing rather than the whole file.
Another great feature was the concept of "quorum". Quorum, as in the organizational term of the number of people present at a meeting necessary for it to be an official meeting of an organization, was the number of reachable hosts necessary to conduct business. Say you had a redundant banking site - and the link between them would go down. If they are a redundant configuration, they would continue to process transactions - with their database quickly diverging. Using quorum nodes, you could set up three hosts on three sites - two major server setups and a simple workstation somewhere central - and voila, no single point of failure.
Besides, there is a magnificent book, "OpenVMS Internals and Data Structures", which so elegantly and wonderfully describes operating system design.
I really, really hope that OpenVMS could be open-sourced and this codebase might serve as the base for a community-written x86 port.