When looking at standards for virtual worlds, you have to ask which of the various clients stand-alone. By this, the original concept was worlds hosted as web pages. Like the early web pages, most of the interaction was client side, so no MU. With the eventual rise of MU, clients have become reliant on the server technology to the degree that playing worlds stand-alone doesn't work.
The exception being VRML/X3D. As a client, it is able to be both a good client side browser and the client of a server. Mundane stuff, yes, but important to the market.
What some who create a VR presence in the MU space discover is they might not like what their neighbors are doing. MU collaborative worlds have the social structures of pilgrims building in the wilderness.
Private islands must be cheap.... say, FREE. It ought to be an option when you sign up for the phone/internet/cable combo whatever t'is.
I want my virtual world as a virtual living room: secure, noise-local, privacy-guaranteed. We used to call that A Home Page.