Richard Bartle (the MUD daddy) has listed a set of the “First Principles of Virtual Worlds” over at Terra Nova. They are as follows:
i) To count as a world, its existence has to be independent of that of the players: it continues to run when you’re not there. In other words, it has persistence.
ii) There are absolute limitations on the actions that can be undertaken in the world: it has a physics. Because the world is virtual, the physics is implemented via computers.
iii) Players operate within the physics: each represents an individual within the virtual world. They have a character (or avatar if you prefer).
iv) Because players exist in the real world but their characters exist in the virtual world, there must be synchronisation. Virtual worlds therefore have to operate at speeds close to real time.
Simple. My favorite is the first. I’ve always wanted to set up a living, breathing, autonomous virtual world. Forget players. Just set up the world with enough AI to let it function on its own. It would be like a virtual zoo. You could log in and watch the world just live. Interestingly, that is also one of the things I like about Oblivion. I like following people around as they do their daily routine just to see what it is.