Raph Koster is an authority on virtual worlds. He's designed several - including Ultima Online and Star Wars Galaxies - and has been on the "implementation side" of countless others since before virtual worlds were 3D-graphics driven.
There's a strange symbiotic and contentious relationship between the 3D-visual virtual worlds (we'll call them MMOGs for the sake of argument) and those without the 3D graphics (we'll call them MUDs). Or is there?
Some people are precious about how different the two are, with MMOG-ists suggesting that their forum of virtuality is better because of its immersive qualities, and MUD-ists arguing that their virtual zones are superior because they don't restrict the user with the visuals of someone else's vision.
I'd like to refer to Raph for his perspective on the similarities between MUDs and MMOGs:
...it's undeniable that presentation has an enormous cognitive impact on the user of a client. Many variables may fluctuate massively, such as degree of immersion, spatial awareness, ease of parsing data, and so on. However, we must not forget that different brains have different cognitive strengths. A common mistake made by MMORPG exceptionalists is that graphics are necessarily more immersive. This is exactly the same sort of logic that says that movies must be more immersive than books. Different strokes for different folks, as always.
The thing we should never lose sight of is that in the end, what the player is participating in is actually the simulation on the server. Humans have an amazing ability to see abstractions behind displays, and to elide out insignificant information. ... The true typology of virtual worlds lies not client-side, but in the differing methods of handling databases, user interactions, and persistence.
Indeed, what makes these spaces truly special are the interpersonal interactions that are fostered with the technology. Visuals or no, the true greatness of this software is that community can emerge through a series of 1's and 0's.