A basic and oversimplified answer would be that Earth has the right amount of heat, both to begin with and a continued supply from radioactive elements. Venus has too much (surface temp. is around 750 K) and therefore the planet doesn't have the same stratification (i.e. cold-hard crust, mantle, core) as Earth. Mars on the other hand it too cold. It doesn't have the same size as Earth resulting in less thermal energy which is the driving force behind plate tectonics. This lack of heat results in very little force trying to move the crust around.
The answer to what makes Earth so special is really it has the 'right' amount of heat. Keeping in mind that the 'goal' is to get rid of the heat and plate tectonics and all that it entails (e.g. mantle plumes, mid-ocean rifts, volcanoes) are ways to release that heat from the Earth. Eventually all these mechanisms will slow down and stop and Earth will be in a similar state to Mars.
As for your main question, I am no physicist, but I would speculate that formation by acceration would be required so that the body was stratified into some form of cold-hard crust, mantle, and core, and there would be some minimum limits on the size of the body (possible maximum limits but I have no idea about that) because anything too small would solidify pretty quickly (or just become a blob of molten magma, would depend on how hot it was and what kind of environment it was in i.e. has an atmosphere or no atmosphere). But as I previously stated the key factors would be the right amount of initial heat (from the accreation of the body) and the correct amount of insulation. Too much of both and you get Venus, too little Mars.
As a side point this article makes the good point that it does depend on what you define as plate tectonics and what we have on Earth may not be the norm... And who knows Venus might have cooled down enough in a few hundred million years that it present Earth-like plate tectonics.