What properties must a rocky body possess in order to exhibit the magnetic dynamo?
Aside from the three requirements already mentioned, electrically conductive fluid, rotation of the planet and convection of the fluid, the main additional requirement is a seed magnetic field.  There must be an initial magnetic field for the dynamo to start.  

See the following reference for more information: http://astro.berkeley.edu/~gmarcy/astro160/papers/Magnetic_dynamos_physics.pdf