It may be that the real difference between what we consider a Virtual World and what we consider a Massively Multiplayer Game is the sort of players that play the games, and how those players are catered to by the publishers.
I recently read an article (sorry, I no longer have the link) that made the assertion that so-called hardcore gamers are far more apt to purchase subscriptions than so-called casual gamers. Casual gamers, it said, preferred to pay using the micro-transaction type models.
While there are certainly examples of hard-core games offering micro-transaction content as well as casual games offering subscriptions, from a demographic standpoint, the division is obvious. Most Virtual Worlds are moving, at least in part, to the micro-transaction model. In just the last year, it's becoming generally accepted, despite the drawbacks, that the micro-transaction model is the strongest choice for a successful Virtual Worlds. World of Warcraft, and most hard-core MMO-type games, have settled on the subscription system as the way to go.
Hard-Core -> MMO -> Subscription Payments
Casual -> Virtual World -> Micro-transactions
No so much a rule, but a guideline.