The obvious result of an open economy, as can be seen in many of today's MMOs, is that the money is not worth much anymore. Anyone can acquire tons of it, which leads to everyone eventually having all the gold needed to buy full plate suits (or the best armor and equipment).

The closed economy experiments didn't have any better results either; in fact, they did even worse because of the obvious trend of gathering the world resources in the hands of a few people who were the first to enter the worlds or played much more than casual gamers.