A metaphor is an analogy which is intended to assist in our understanding of something by indicating points of similarity between them. Our virtual worlds are all metaphors to one degree or another. Many artificial constructs are.
The problem with metaphors is that they're generally not readily reversible, and it's far too easy to get caught up in them and stretch them far too far.
Water is often used as a convenient metaphor for describing the behaviour of electricity in electric circuits, and the effective operation of individual components. The similarities of water as a metaphor for electricity make a lot more sense than the similarities of electricity as a metaphor for water. It just doesn't work that way around, and we're all far more familiar with what water does than we are with what electrons are doing.
In an MMOG, the world is largely a metaphor for the game mechanics. In non-game virtual worlds, the world is a metaphor for the underlying communications/publishing platform. The metaphor is at least partially immersive and leverages what we already know, enabling easier transition and use of complex underlying systems.
Metaphors are, however, seductive. Their simplicity and obviousness urges us to adopt them as complete models, even though they are only partially applicable. In LotRO, there's probably nothing on the Southern side of the hills bordering the Vale of Andrath. Just void. Middle Earth is just a thin shell in the middle of substanceless void. Everything that exists is there simply to embody the game-mechanics and foster the illusion.
Being a fictional world of constrained game mechanics, it's difficult to mistake Middle Earth or World of Warcraft's Azeroth for anything apart or separated from the physical world that we all live in. They're pockets of product in our regular world.
The non-game virtual worlds get a bit hazier. Second Life allows great freedom in content creation, and in speech and behaviour (though not completely unrestricted), as well as having a currency and a lot of trade of that currency for goods and services. Does this mean that Second Life is a distinct and separate world from the physical one, and that it has its own economy? Well, no. Not really.
As a communications platform represented as a 3D world, Second Life is as much an extension of the physical world as the Web or the various IRC networks or the US Postal Service. As for its economy, it would be fairer to say that the economy of Second Life is simply one channel of the physical world's economy (yes, the physical world's economy itself is rather virtual and insubstantial). Trade between users is part of the physical world economy - part of that trade takes place in Second Life, just as portions of trade between people might take place on eBay or via Paypal. Virtual worlds and MMOs are markets, rather than economies per se.
The physical world, the economy, the law - none of these things go away or magically stop being applicable to you when you turn on your PC, or log in to your MMO or virtual world of choice. Believing otherwise is stretching the metaphor too far.
Yoon also talks about taking the metaphor too far, and simply appears to view Second Life as a product designed to be appealing, functional and popular - and that's not taking the metaphor far enough (but does rather explain a lot). In reality, the real position is likely somewhere in the middle.
Ultimately metaphors are useful, and it behooves us to use them to facilitate use and understand - so long as we don't overdo them. Conversely underutilizing the metaphor that developers and product managers create for the users, leads to a widening of the gap of incomprehension between the users of a world and the operators of a world. Compromise is ultimately required to prevent eventual fracture.