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In my experience people will do a better job if the chance of player interaction increases, so having more builders than players will be a total disaster.

Next you can apply the theory of general intelligence to assume that more intelligent players will create better content. People tend to pick leaders with IQs one to two standard deviations above the average (115 to 130) as people with too high IQs are a little too different to socially connect with the masses. If you use a clan system and let only the leaders build and the average IQ on your MUD is around 105, that should give you some 120 to 130 IQ builders, and research has shown that a 130 IQ person is capable of handling anything you could throw at them, while a 120 IQ builder is easily trainable. Another advantage of using clans is that it enforces a healthy builder vs player ratio, and clan members might check out the content just to be nice - increasing player interaction.

Another trick is giving building a steep learning curve, which would discourage dummies. Adding a scripting language with a low level interface to the MUD might give better results than an easier high level interface.

The only downside of very high IQ (according to the latest theories) is that while it increases creative output, it comes at the cost of latent creativity. This theory will likely gain popularity in a couple of years when the true implications become clear to the mainstream. The current estimate is that creativity peaks (on average) at 120 IQs.

So for the best creative and original work I would advice finding 120 IQ builders, with a 130 IQ head builder (who more often than not is the main coder), and a builder to player ratio of at least 1 to 10, though I think you can work with lower values if content generation is slow so the player base remains excited about new additions, so you'd want to find some way to slow down the building process considerably - scripting and extra descriptions are good ways that I know of.