What's in a good model

After reading Joshua M. Epstein comprehensive explanation of why we make models, I asked my self which elements does a good model contain? and well this is my list:

• Prose: because the use of a informal human language creates a mental image of the system.
• Illustration: because a drawing acts as a map of the system.
• Numbers: because numbers, originating from either measurements or assumptions, are inherent to system and they can be calculated up on.
• Algorithm: because to make even the simplest algorithm that describes a system the connections within the system has to be identified.
• Mathematics: because mathematical notation demands formality in thinking.
  

The spectrum of systems

I have read an article from New Scientist, about the financial system viewed as an ecological system. It made me think about heterogeneous versus homogeneous systems as the end points of a spectrum; where:

• Heterogeneous systems are robust but not optimal in an economic sense.
• Homogeneous systems are vulnerable but optimal in an economic sense.