You see a book on the ground. Thus, a number of people could be standing in that field with you, and as you decided whether or not to take the book, someone else could take it first, and run away. You could then 'go left' to catch them. One of the things that made MUDs so popular was the fact that they were not static, but dynamic. If you didn't like the field you were standing in, you could build your own, and write the description, determine who could come in and out, create your own objects, your virtual home.
People set up their own areas, and most MUDs grew at phenomenal rates.
Same old, same old. So now that MOOs are here to stay, and thousands of people are accessing them every day, we must ask ourselves what these virtual spaces are good for. It keeps your hair stiff. But all joking aside, let's look at what a MOO really is. A MOO is a virtual space on the internet, as described above.