what else would need to be massively multiuser other than games?
A database.
I always thought the "Online" in the MMO tag suggested that it required a connection to an external server as oppose to classic multiplayer games (e.g. Quake 2) which you could create a server yourself or even LAN.
Concept is really the same either way in a technical sense. If you create a server yourself, it's still a server. Even if you connect to and play on a server on the same computer that is running it, it's still a client connecting to a server. And everyone else that plays on your server would be connecting to a server external to their PC whether they are out in the cloud or in the same subnet. Besides, almost all worthwhile game servers are hosted by hosting companies that actually have decent hardware and data pipes to handle online games.
The only real difference is that a company with a MMOG generally runs all the servers, while other online games have servers that can be run by pretty much anyone.
In essence, I agree that the "O" in "MMO" is redundant.